The biggest are EMS and BiT being patched on HD. Map Glitch is also patched but also wouldn't be used I think with current strats. There are other differences like HD having less bugs to kill in the twilight sections

You cannot stop the timer at any time with the current ruleset. There are longer cutscenes (such as Knight fight) where you might have time if you're fast enough

Why is that? You have tried this link right? https://discordapp.com/invite/hollowknight and you haven't been in the discord before?