What the title states.

Why bother to split the executable at all?

There's technical reasons, like breaking the netcode. There's also gameplay reasons such as the original boxed version still works on battle.net. PvP is included. Having high resolution and standard with each other gives one side an advantage.

I don't know what this "HiRes" thing is about... because this GOG version just upscales the game from 640x480, like any graphics card or monitor can do.

I'm confused about the Resolution change too, it's looks the same. the Belzebub HD mode adds Ultrawide Support allows zoom. GOG or Blizzard need to get a hold of this modder and get his resolution & zoom changes in. Provide a slider in the MP options for the maximum zoom allowed in a session.

Checked on Blizzard site.
Seems to be a Blizzard decision and Blizzard refers to GOG for answers on this version. As a Blizzard decision cannot be answered here, I guess we are high and dry.

You're right about the ddraw wrapper being the problem. As part of its security checks the battle.net code reads from the framebuffer to check if the logo was drawn properly, if the wrapper did proper emulation (i.e. supported reading and not just writing) it would pass.

You can play battle.net with the DX wrapper no problemo. There's nothing preventing you guys to run the wrapper with the standard diablo version, which is basically the same thing described in the guide for running the Hellfire expansion.

The issue is that wrapper doesn't play nice with their battle.net UI, so all the texts and buttons have invisible descriptions in fullscreen, and in windowed mode the texts are off-screen. But if you know where to click, you can still click the buttons. I can log into battle.net with the DX wrapper, there is no code that would prevent it.

какое игровое преимущество? внутри игры все одинаковое, инвентарь, радиус обзора, версия игры
внутри никто не поймет в какую версию ты играешь, так о каком преимуществе идет речь?

i guess they just can't be bothered to fix the old battle.net UI, or the code is too old to fix (needs rewriting from scratch).

like they said, it is possible to use GOG's DX wrapper with classic Diablo and play on bnet, the bnet UI will distort to a side, but still clickable.

You can connect to Battle.net using the GoG dx version by just copying BATTLE.snp into the dx folder. And adding the connect-forever server to your registry {or patching Storm.dll to use the same key the 'classic' version does} As the actual battle.net GUI is controlled by Battle.snp, it'll be weird and off in the corner. But it works. I'm sure if someone cares enough a patched battle.snp could be created to better align things.

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yes that would work i guess, though it involves registry editing, or setting up a cmd file to do it like GOG does in directplay.cmd. i am glad they named the subdomain connect-forever though, may D1 never die..