I would give him very strong range. Top pairs ds-ss well connected. Something like AKKQds up to JJQTss. In this case I would fold our hand. Utg is probably limping with trashy A high suit. Loose player got only God knows. Our preflop equity is very low.
That's my opinion

I wouldn't consider 26/21 to be particularly tight.

I would guess to raise 2 limpers with 4 high cards or some double suited middle stuff.

A9TJ with a suit is a slam dunk to play this hand in my book (not to say it is correct!). I like these hands with a perfect 3-straight that can flop monster wraps or combo hands

When we are multiway we prefer hands that can make absolute nuts. You can flop nut straight,but you need to check the flop. Specially when flash draw is present. Even the Phill Gallfond recommend check flop with nut straight on some flops. We can be freeroled with someone holding same straight with flash draw as well. Because we are 4 way all possibilities are there and we can get just thin value from river if someone will call with set or two pairs which is unusual 4 way.

With standard stacks, I call in sb with AJT9 suited to the J here with these reads/assumptions. (suited to the A, I am always continuing in one way or another here, obviously

AJT9:Jxx is a good candidate for a 3-bet here, I personally would never flat this hand in this spot as it has terrible multiway equity and you are in the worst possible overall position. 3-bet or fold are the best options.

Tough decision. UTG could be trash AA looking to see a flop cheap or hoping someone raises to come over the top and get most the stacks in pre-flop. What did the CO have?