1) @DonAKAUnq told me about Vluzcan's HL Tools, which add an entity called func_detail that's basically a func_wall but a lot less expensive! Don't count on it, but it may or may not allow my original plan of having the final Somos fight in one long sequence without loading times. But we'll see (that's a nice-to-have anyway).
2) I searched for the source code for SoHL 1.8 and yeah, there it is. Haven't tested it yet, but if it compiles, I can probably fix a few problems programmatically. That's very promising :D It may or may not allow implementation of steam achievements, but I won't get my hopes up for that just yet.
3) Starting Half-Life 1 (the WON version) on Windows 7 has all sorts of weird side effects, like the color scheme being changed and all sorts of scary flickering. Also, no widescreen or window support :D So I changed my test batchfiles to start the Steam version, which is the target version anyway. So, goodbye forever, WON HL!
4) Mr. Evans recommended I look at what the team of Master Sword does. Apparently they have a scripting thing set up to spawn completely custom monsters, with sounds and all. I'll do that, but I think if my source code compiles, I'm good, since I'd only need alterations for two monsters.
5) And yes, I've set up a github project, which only saves the rmf files and my compile batchfiles for now; just for easy backup purposes. Since the limit is 1GB on github, I'm not sure how much of the project I want to add here, but that's something I'll decide later on. :P I still have my local backup, anyway.
I've also tested the mp3 fmod stream and it's just as I thought: The mp3 plays "everywhere" and only stops if it ends or if you trigger another fmod stream. So, it's not exactly what I'd need.
Next, I probably want to try compiling HQS maps with VHLT and then slowly start bringing in HQ maps. We'll see!