I noticed that Cubeface was able to bypass killing the second guard, which is normally required to get out of the first room, by hitting it around five times and squeezing through. I haven't seen any other runs do this successfully and I haven't been able to recreate it in SVE. Was this just really good luck? Or is it only doable in ZDoom?

Okay, so you CAN replicate it in SVE. It requires some luck though, and you have to wait around a bit for the guard to move out of the way, so it's unclear if it's much of a timesaver without the absolute best luck.

I think the main problem is that in ZDoom the guard can still move while being stabbed, while in SVE the guard freezes in place when stabbed.