I'm a bit confused reading all the new patch info and user replies and just want to know is SWBF2 offline version still same one from 2005, offering option to play on dedicated servers, LAN and co-op? New patch hasn't been applied to any files there, only on official Galaxy version, right?

Very first setup from GOG
setup_sw_battlefront2_2.0.0.5 (dig sign from jan 2015). This is proper retail 1.1+DRM-free (although international dvd 1.1 patch already drm-free if start from battlefrontii.exe and not from swbf2 launcher).

Than in 2017 it be some update for offline build.

Later in this year it be first MP update and first offline update. Offline than be rolled back to 2.0.0.5 setup marked as WinXP compatible. Than updated to MP again with MP2 update next to Galaxy MP2 update. Than offline been updated again. setup_star_wars_battlefront_ii_1.1_multiplayer_update_2_(17606) This is pre-latest offline build available before current one. And latest version probably good for offline singleplayer game only.

Than it be updated to version labelled as 1.1 - but that wrong. It still be based on MP version upgraded and downgraded (instead of real revert back to 1.1).
Most important thing that this rolls back for offline installer - Disney+GOG messed with files. Installer downloaded now come with 1.1 exe (wrong one, not retail-compatible) - but with 1.0 files from retail. It even not contain some changes from 1.1.

So. Back to question about original multiplayer, DS and etc. For co-op campaign. For all that you need real 1.1 version.

And that mean you probably bee good with second offline build (first one from 2017 and last to MP updates)
setup_star_wars_-_battlefront_2_1.1_(10935) (do not confuse with currently available installer - sometimes even with same names they differ and here you can see - this ones not same by names\version as well)(this one sign is from 2017)
This is should come with proper 1.1. But propebly something can be worng with files. Or installer itself.
Because GOG did one rollback for offline installer (all else updates - they not rollbacks, all are new - check digital sign) and when they did that - they roll back to very first installer labeled it as WinXP compatible (first available as second alternative installer than been main-only-one downloadable than been updated to MP2).

And again,very first is setup_sw_battlefront2_2.0.0.5 - this one is 100% compatible with any oldest mods, MP (including play with CD version owners), MP DS and everything else. I have it installed right now.

Try to duckduckgo-it magnet link for that version. Cant post it here (i know one magnet link but no goodies), u know. Just make sure it have proper digital sign in installer file properties, and do enable integrity files check when you configure installation.
Should be this files:
sw_battlefront_2_manual.zip (2,26 mb)
sw_battlefront_2_wallpaper.zip (635 Kb)
setup_sw_battlefront2_2.0.0.5.exe (29 mb)
setup_sw_battlefront2_2.0.0.5-1.bin (3,9 Gb)
setup_sw_battlefront2_2.0.0.5-2.bin (2,84 Gb)
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To play MP on such old original version - you need GameRanger. If you googleit some other solutions - forget about them. One is 100% fake, another ones not needed anyway, especially if they ask patch game to something unofficial. GameRanger work "as is".
Or just find someone to play LAN\Co-op.
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Of course to play on current GOG Galaxy official multiplayer service you need GOG Galaxy or Steam version only.

Oh, this was a bit hard to read but I got it-I think. Never realized it had so many updates. So, setup_sw_battlefront2_2.0.0.5 contains all OG files and can be played through GameRanger. To be sure- GOG offers this version for download, right? I didn't buy the game yet so that's why I ask, needed to be sure.

Torrent only.
Current installer is setup_star_wars_battlefront_ii_1.1_(10935) (sign from Oct 2018) (looks almost like installer from 2017 but they differ)(although i cant say for sure which files in 2017 1.1 build, but installers differ for sure). Maybe Disney+GOG want revert it back to 1.1 (as labelled), but something wrong with files - it not compatible with 1.1 game version and some 1.1 changes are missing.

http://www.gog.com/forum/general/the_what_did_just_update_thread/post20984

Ok, tnx for clarification. Figured as much but I needed to be sure I got everything right. So can you confirm version 1.1 (10935) digitally signed Oct 2018 (offline installer) has features I asked- that is LAN, co-op and can be set up to work through GameRanger (or other alternatives)?

2.0.0.5 - to be 100% sure.

So only 2.0.0.5 is right one but like you said-that one needs to be pirated. Too bad they didn't keep everything as OG.

To be 100% sure - 2.0.0.5 Exactly this one and not second one pre-MP builds be available as legacy version. It be available twice - as main build revert back to 2.0.0.5 and as alternative installer. Now it not available officially - so yes, only magnet links. Alternative installers is very rare, unfortunately. Even for originally DOS+Win games.

Also there is Fallout Classic for first three games - but they installers got Interplay->Bethesda updates too. And got EU censored version instead of real Classic ones. It stil lbe avaialbe as Classic for old owners, but not Classic and at same time outdated non-Classic Bethesda version that not include HD resolution mod. not a problem if it change publisher logo in game\launcher - but it based on censored EU version, and original one is removed entirely.
So real Classic ones - but very first one only - as available from torrents as well as Battlefront 2. But for SWBF2 you can try find every one offline builds from GOG since 2015 year 2.0.0.5 - and they all should have proper digital sign.

Fallout;s latest Classic builds is missing from internet - only Mac-emulated version have it. Or oldest 1.0.0.0 GOG build from 2008 year (no digits in installers and no digital signs back than, so dangerous).

It is safe to download SWBF2 2.0.0.5 build from magnet. Installer have integrity files check for -1.\-2.bin files, and setup exe itself have digital sign GOG Limited 20 jan '15 (crc32: 2E28F95F). Russian torrent-forum rutracker have magnet link to it - but no optional goodies, just installer. http://www.gogdb.org/product/1421404701
(wrongly shown in GOG db as Patch - this is installer and previously GOG db can show alternative (eg x32 for mainly x64 game) installers as installers, not as patches)
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Alternatively, you can use GOG Galaxy. This is not DRM-free, currently GOG Galaxy require Win 8.1+. But exe files launched directly should be DRM-free. There is tweaks to use Galaxy on Win 7 Sp1. So it almost good as offline. Not properly DRM-free, but almost it. I mean, using GOG Galaxy you can try install oldest version - but this one method probably not a good way: there is chance that oldest available is only
50028763532095574 -- 1.1 - this one is latest pre-MP version same as offline one - second to 2.0.0.5. From 2017. And this version probably not good itself even if it be pre-MP update (as i guessed in posts above).

You can try this as alternative to magnet links to real 1.1 2.0.0.5 build. After all, magnet links itself is only option after buying game. Right? In case if it be correct 1.1 really WinXP\1.1 retail compatible.

Or you can just play GOG Galaxy\Steam 1.2 (they equal) current online versions. They not compatible with oldest mods, old Dedicated Servers. But they have new online service and old singleplayer. At some point of time there been more players than on GameRanger, but that probably been temporary phenomenon...?

Or again, to be 100% sure and to get exactly retail-compatible 1.1 - setup 2.0.0.5 from magnet. it not piracy to use torrent\p2p even if "corporations" and your ISP say it (Valve shared Steam on torrent file and even ENTIRE Valve back catalogue games! old moderators probably still remember that...). And especially this is not piracy if you buy game. Im buy it both on Steam and GOG. And i dont care that i dont have retail edition in my country - this is why we have GOG and Steam!

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By the way, there is Steam updates in February, partially reverted. Seems not for GOG Galaxy, although Galaxy is main version and Steam based on it and use Galaxy as online servive.

It not problematic. it just lamers. Ofc update will broke some old mods. And for other things... controllers, vsync, resolutions, FPS lock (always should be locked for every games - just differ FPS for each game) - player should correctly configure system and game. This is players job, not devs.
Moreover, old mods require 1.1 which one never be available on Steam and GOG Galaxy since first MP update. This year update is not 1.1->1.2, it been 1.2 since 2017. This is 1.2.1.1 revert back to 1.2.1.0. or something like that. maybe 1.2.1.1 is current version (GOG Galaxy didnt get this to public) and that february one is some more actual.
It not revert back - it still version from February - not all files reverted. But GOG Galaxy dont get that update for public.

http://www.gogdb.org/product/1421404701 - check build and changelogs, also check build API section - this site show it sometimes wrong (this one is showed when in API there is no 1.2.1.1 - so it probably shortly available in public API then removed - and site keep it as public one)(usually there is more public updated and site show only some of them randomly).

Sorry for the stupid question....

If I'm only interested in the offline installer, and only interested in playing LAN games, will the latest version work, or do I have to download a particular version from gog to just play with 1 other person on my own LAN?

technically everythign will work. but current version is downgrade 1.1->1.2->1.0 with files from every version and exe not compatible with old 1.1.

if you an friend want easy way - probably it is ok for LAN.

Does this game have online multiplayer?

Yes. For Steam and GOG Galaxy versions.