This currently won't compile on my part (machine, using Windows 11), mainly because of the broken includes; I'll probably fix the include's by tomorrow. It may compile on OSX/Linux but not really sure.

Wouldn't the x64 support require the singleplayer SDK base to have x64 engine binaries as well?

Wouldn't the x64 support require the singleplayer SDK base to have x64 engine binaries as well?

no, as the multiplayer base itself supports singleplayer code. iirc before the tf2 sdk update, some singleplayer mods used the multiplayer branch as it was a newer branch at the time.

singleplayer mods can safely use the 64 bit code as long as they mount episodic content from the singleplayer base and use the multiplayer base for launching

no, as the multiplayer base itself supports singleplayer code. iirc before the tf2 sdk update, some singleplayer mods used the multiplayer branch as it was a newer branch at the time.

If x64 HL2 mods can already be made with the MP branch, and the only option to support them is with MP binaries, then why should these changes be ported to the SP branch in the first place?

no, as the multiplayer base itself supports singleplayer code. iirc before the tf2 sdk update, some singleplayer mods used the multiplayer branch as it was a newer branch at the time.

If x64 HL2 mods can already be made with the MP branch, and the only option to support them is with MP binaries, then why should these changes be ported to the SP branch in the first place?

I believe VALVe will update Half-Life 2 and its episodic releases with support for x64, and if they do not, this pull request was also made for VS2022 support.