Yeah if I did decide to speedrun this I would run it on the easiest difficulty for the reasons you've already stated. I've beaten the game on veteran, which is a pretty tough challenge IMO so I'd say I'm fairly good at the game, (and it doesn't hurt to note that I always do good online on hardcore TD

) but I've only
actually ran through about the first half of the levels in the main campaign so I don't know much about running through the last levels.
I've looked on youtube for glitches but I haven't been able to find anything for the main campaign, (although I haven't looked
extremely hard) but like you said, I've found tons of videos for glitches on multiplayer & zombie maps that have or haven't been patched.
Now I'm not good at finding giltches...at all. That's just not the way I think. I'd say I'm pretty good at running and stuff like that, I'm just not good at finding things.

So if I do decide to run this, I would need quite a bit of help finding glitches.