Brock
In reality all of those rib protector manufactures you mentioned are all good.
As for comfort goes, it is hard to tell, as what is comfortable to you may be uncomfortable to others, the main thing is to stay safe, and being able to move somewhat freely, and staying wedged and packed pretty good inside your seat.
Those bulky vests are very helpful once you get used to them, another thing to consider is that rib cage bruising and small hairline fractures caused by G force and being tossed around and bounced in the corners are a lot more painful and uncomfortable.
The answer to your other question is: Ask your club what the proper age it is for your kid to stop wearing a chest protector, some clubs are different than others, but honestly I would say wear it no matter what, for his own safety, you just never know when it’s gonna happen, but when it does you want him to be ready to absorb and insulate the impact of a steering wheel or someone else’s tire or chassis.
Now let’s move on to padding on the seat. If he likes it the way it is, then leave it alone, but if he complains of pain on the sides where the seat strut bolts and washers are, then either get a 1/4 padded seat or fill it up with patio floor mat or boat carpet and padding.
Those are my 2 cents. Hope that helps. Fast Freddy. ;)