1 - you can only get gta v legally on rock* social/launcher
2 - yes
3 - no idea, I don’t play multiplayer
4 - base is about 65gb, you Amy want redux mod though:
https://gta5redux.com/

also does the rockstar launcher have a web browser like steam in big picture mode i hope not

I believe it doesn't also about how much space you need I believe it is more than 80gb if I remember correctly and with all those multiplayer updates it could be more.

TBH il just get another SSD for backups and gtav installation

you can also get it legally on Steam Would be nice if it was just 65 gigabytes still, but that number hasn't been true since 2016.
Currently the game fills 89 Gigabytes

And of course for mods the warning is: If you install them, you can't play Online, since it would just get you banned, even if it's SP Mods or just graphic mods or anything.

1 - does not matter where you buy it,if you buy it on steam it still requires rock* third party account, hence you can only get it on rock* social even if you pay via steam.
4 - yes, I only know the base game size. It has obviously increased substantially since then.

Not to be a killjoy, but why not ask on the Steam forums where people are morel likely to know the answer to your questions?

What i read somewhere,
whole installation is around 100GB!