BoistopmotionTo clarify, Pre 07-25-23 demos are working for you now?
Seemingly so, the only things I would have changed was I tried killing steam after starting tf2 to see if demos would play then (they didn't, it crashed), and I also tried replacing steam_api.dll and steamnetworkingsockets.dll in the old version with the versions from the latest release of tf2.
I reverted everything I did, yet somehow demos play back fine now without crashing tf2. The demos I tested were the mastercomfig benchmark_test.dem and benchmark1.dem from the tftv benchmarks thread, as I knew both of those are supposed to be able to play in this version of tf2.