Companions gain XP with the inquisitor only if they are in the active party, but those not taken will be automatically leveled to one below the inquisitor if they fall behind, and this is reflected as soon as they are recruited in the field or upon returning to Haven/Skyhold with the respective ability points. This has been the case in every DA game. Whether or not the whole active party gets codex XP and such, I couldn't say, but no companion is ever more than one level behind the inquisitor and they always get an ability point for leveling.
HOWEVER, I returned to an early save file and confirmed some things that may have answered my question. For one, human companions do get an extra ability point at level one, BUT NOT COLE. Presumably this is because he is not strictly "human", and he doesn't get one even if you choose to make him more so. Second, everyone begins with two ability points at level one before the human bonus point. I believed it was three, but this was incorrect. So my memory must be off about whether or not I got an amulet for Iron Bull this play-through; I believed I did not, but if I hadn't he'd have only 28 points. He has the same as Dorian because even though Dorian did not get one (the only one available for him requires completing Champions of the Just), Dorian is a human. So the only companions who can reach 31 points without exploits, because depending on your choices there are two amulets in the game available to them, are Cassandra, Blackwall and Vivienne. Two are available for Cole, Varric, Solas and Iron Bull, so they could reach 30. Only one amulet remains in the game for Sera, so she can get up to 29, and one remains for Dorian to get a max of 30. Most of your characters will have 29 or 30 points between human bonus points and amulets.
Hopefully anyone who sees this finds it informative lol.