I have my mouse set to fire groups 1 and 2 not the selected group.
I have also noticed AC5 rounds going through a hunchback with a friend spectating me also seeing it.
I've experienced the "I push the button and it doesn't fire till I hit it a second time" glitch. I sometimes have to hit the fire button twice to get a single shot off. Particularly with the first round of LRMs I want to shoot off.
I've also had a round go through a mech before. It was an AC 20 shot too. It went through three mechs in the same match. Haven't seen it happen again, unless you count when I happen to somehow shoot through a mechs armpit... I've had plenty of near hits like that. But that's very true to the books.
I've noticed it too. Sometimes, it hits where my mouse was clicked over when I clicked, which can be great when a shot seems to "home in" from my AC 20. But normally my shot is delayed from when I pressed the fire button and it shoots afterwards, way off target now. That delay could use to be shortened or removed. I don't experience such a delay with lasers.