The trouble with making that fighting style specifically rewarding for parties of phalanxes is this: having another phalanx around is not a situational bonus, usually. Either you'll have other players who took this option from the start with the intention of exercising it all the time, or you won't have anyone. If it's balanced to be competitive when solo, it will be too strong in a phalanx party. If it's balanced for a party, it won't be very good solo. In all likelihood. The tactical consideration of "you must be next to each other" is fine, since that's not a given, but you can't leverage the restriction that they must be phalanxes to buy more mechanical effect, since that will usually be a constant, not a tactical consideration.
Getting a total of +3 AC from the Phalanx version, or potentially more from the Survivor version, seems way too potent for a fighting style. It's already possible (with a lot of effort) to break AC, such that only critical hits from most enemies can touch you. Adding more miscellaneous bonuses, particularly sizable ones like these, makes that easier. The reason I went with cover is that it doesn't raise the optimization limit, it just makes it easier to access an effect that already exists.
I have to agree, too, that a flat AC boost that starts at 1 any time you have a shield mostly invalidates Defensive. A shield is already a valuable trade-off between offense and defense, so it doesn't need extra boosts just because it limits options (it's not limiting anything, for a character who already intended to use a shield, to take an option that benefits shields). Nor is it more rewarding, for the same reason. Extra incentives to do something you were already going to do are just freebies.