com_singlecore doesn't work
There is no such cvar, so obviously it doesn't work.
and sometimes i get virtual memory error and the game crashes
Using too high hunkmegs is bad, because then there is no memory left for other allocations. Keep it in 600-800MB range and you will be OK.
I also can't hold down a key to spam a command.
i get invalid game folder and disconnected from the server each map cycle
Not sure what these are about, I haven't changed anything in keyboard code. Pure server pk3 check is exactly same as in official client.
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The refreshrate is still broken higher then 60hz is not possible.
Must be something on your setup. I'm playing on CRT @ 100Hz refresh without any problems.
You can try using
ReForce to change default refresh rate.