I just think that it's clear the game was written by people who are more in Niara and eye's camp vis-a-vis plane to Antartica, plane to New York, so in that light Gale's choice is intended to not be a bad one or to reflect badly on his character. Though then you get into a whole thing about authorial intent and death of the author, that's a whole rabbit hole.
As for Gale getting to the Astral plane, I really don't know how difficult it is to do that. I had the impression he's been living with this for like, months. I honestly haven't given as much thought to what he could have done to figure it out and in hindsight Gale probably could have figured something out. What? I'm not sure, I lackthe lore knowledge (for instance Szass Tam? No idea who that is) but I also assumed that when Mystra left him, that was what knocked him back to a first level wizard and he's been like that ever since. Maybe the timeline is that he was apart from Mystra for several months, possibly even years before finding the orb, which was more recent. I don't know, I'm just speculating.
Speaking of speculation, I initially thought your suggestion of him worshipping Shar or having a connection to Shar was out of nowhere, but then I thought about it. There are a few lines he has with Shadowheart that suggest... something. I don't think he's a sconnected as you theorize (I think it would feel weird if we had two disconnected Shar-related characters in the story this way) but I can believe there's SOMETHING going on with that.
