ExInterlockedPopEntrySList does not load properly for me in IDA, I cant see graphic view, so this comment is using your ExInterlockedFlushSList as example:
goal is to replace 0F C7 4D 00 "cmpxchg8b qword ptr [ebp+0]" with your "cmpxcng_486 ptr [ebp+0]"
assuming that 4 bytes not enough space for jump, so needs to steal space from previous code "mov cx,bx" 66 8B CB
so existing code replaced to jump new location, and NOP for rest. 66 8B CB 0F C7 4D 00 -> E8 xx xx xx xx 90 90
then in new location add the stolen code, then whatever your code is for cmpxcng_486 then return. 66 8B CB xx xx xx xx xx xx xx xx C3
Or working example for a different DLL if makes more clear https://postlmg.cc/Lq8XdxSf