set_pcvar_* works fine except when you type your cvar in console. It because how pcvar worked. There is a changed in AMX Mod X 1.8 addressed the issued.
If you did server_print("My pCvar Value: %d", get_pcvar_num(the_pcvar)); It will show you the correct value.
P.S. Don't ask when AMX Mod X 1.8 is going to be release. It will probably be done when the Dev believe it ready.