What I meant specifically is that Gale never has any unique interactions until Act 3. Karlach has some fun introspective moments, Shadowheart has a fair amount of scenes in Act 1 and LOTS in Act 2. Laezel has lots of things to do in every act, because of the GIthyanki plot. Gale has the Tressym scenes in Act 1, and that's it. Whereas, as you take him as a companion, he's literally the exposition character that discusses and explains the main plot.
And the Shadowmagic spellslot thing in Act 2, though that feels minor compared to what you say about Shart. I was surprised that the people who comment on the strange magic of the tadpole don't notice that the whole wizard is full of this strange magic. Especially Aunty Ethel has such a strong aversion to the netherese magic that I would have expected a stronger reaction - even more so since she does have a mockery line about the wizard being rot and ruin.
I think that is a special service for Gale. Unless this was changed in the recent update, I think she takes him to Elysium right after restoring him (I only saw videos of it), thereby removing him from the tapestry of fate as Withers puts it. This is also what she promises to do should he explode. Only if he explodes, Withers makes a grab for his soul before Mystra can make good of her promise because Gale apparently still has stuff to do.
I see what you did there.

A Gale thread needs puns! I just had him comment on Clowns being 'armless when finding Dribbles's arm.

Or you can both be ambitious wizards, supportive of each others' plans and wreak havoc together. Fun times.
That is of course very true. Please hunt down all the rogue shadow mages and then pilfer their secrets to your heart's content.
Judging from the epilogue chat with Halsin, I'd even say it's canon.
If only we didn't need a damn mind flayer at that specific moment. This is the only reason for me to recruit her. I don't want to have to sacrifice my chara, Gale or Orpheus. Basicly, I'm forced to have her in my pt to have my desired ending.
I am very content with shrimping Orpheus because it allows the Githyanki to forge their own path without any godlike kings or queens but then again, you can't help feel sorry for the guy.