What actually counts as glitches, other than the mentioned piggy getting stuck?

any form of clipping i believe

using or taking things through walls maybe

Does pressing a key at the beginning of a round to gain like 3 seconds of headstart count?

nope

edit: its not a glitch i mean

um why is there a two player run at 1 player category on mall? speedrun.com fix this please, put this run on duos

fixed