Glad you enjoyed this Day of the course!
I would shove 22 here against almost any opponent
. With AA, I would shove against a good player to keep him guessing, exactly like you're saying -- the problem is that a small 3-bet would likely look too strong because I'd only be making that play with monster hands. So it's better to be unpredictable against someone who understands that a small 3-bet here represents a lot of strength.
: As a general rule, if you have a hand that is more than strong enough to call a raise, but you wouldn't want your opponent to 4-bet you, then you shouldn't make a light 3-bet. With that A9o, it's not quite strong enough to continue facing a 4-bet against most opponents, but also too strong to want to fold. So it all comes down to the specific for which hands you pick. A9o might make a great light 3-bet if you can easily fold it facing a 4-bet, but blind-vs-blind it's too strong to normally use in that way.
. There are definitely good players who incorporate open-limping (when first in) into their strategy from different positions, but I recommend only doing it from the small blind. Stay aggressive and give yourself a chance to win the pot pre-flop
Great questions and I hope this helps!