First, I suggest returning your seat
The Alpha Omega has lower top usable harness slots: the actual top slots are for
booster use only. Too many kids outgrow the harness before they're ready to use a
booster correctly & besides, there are more dedicated boosters available. Too many kids also outgrow the AO as a
booster before they're ready. This seat is advertised very well, but this seat has an increased risk of misuse because of the top slot issue. Also, 3-in-1 seats are considered jacks of all trades, masters of none. It will expire before your child is able to safely ride without a seat. IMO, the AO is generally good for
RF, but poor
FF. I personally would feel okay using it
RF, but my child outgrew the harness
FF the same time she outgrew it
RF -- 2 years old -- long before she was capable of using any
booster, especially 1 without deep head wings.
Will you be uninstalling/reinstalling 1 carseat from car-to-car frequently? If so, I suggest buying 2 seats to keep them installed which can help reduce the risk of misuse. What is your budget?

Also, make sure baby has at least 1" of hard plastic shell above his head in the
Keyfit. If not, then he has already outgrown it.
As for the issue of deep vehicle seats, a pool noddle usually remedies this problem. Have you
contacted your local technicians yet?