Yeah I get what you mean but at that point I attribute it all to the writer. Bubble made the character and the quest and didn't account for all these questions we're bringing up because...idk, reasons I guess.
Quinn's older quest itself is pretty nonsensical. I think the reasoning given for MC needing to buy the stuff is because Quinn is in cultist robes and doesn't want the villagers to see him and maybe a little bit of him testing to see if we'd help him/he could trust us. But like...no one had a cloak he could use to go on his own? You're right that someone escaping from a cult would probably want to arm themselves and find - if not a safe haven - then people he can trust to watch his back. And if you're running from a cult, tho the MC is strong, they are actively working against and thus always interacting with the cult so, maybe not the best group to latch onto if you don't want the cult to find you lol.
I was mostly joking in that post, but I do agree with you that Quin could be a better character, if he was fleshed out, or certain aspects of his character was explained. For example I've always seen him in a similar light to Zevran for DAO. A character that I didn't really like at first, and saw no reason to trust. I pretty much killed Zev in most of my early play throughs of DAO, and even in the few instance where I let him live, I barely interacted with him. (Surprisingly he's never betrayed me in Denerim.)
But, when I finally decided to see his content, I realized mostly stats wise he's better than Lelianna imo, and his story isn't a bad one. A good way to fix Quin seeming like an idiot for joining the cult would be to say he was born into it. Maybe Tollus has a harem and Quin is born from one of Tollus's many lovers. Maybe the cult kidnapped him and raised him in it. Much in the same way that Zevran was forced to be an Antivan Crows. Anyone of these options would work for me. It still gives the player a reason to distrust Quin as his story could be complete bs. But, its less stupid then "I joined cult cause I thought would be fun, and then realized I was wrong."
Next if were going with the alternative story I'm suggesting it would explain his personality. Both version would explain why he's so hypersexual (I know the current story already does it, but I still feel like more could be done with Quin besides I just really like sex. We don't need two Caits) If he's one of Tollus's son's maybe he put on a pedestal and treated like a god. Getting to have sex with who ever he wants because people don't want to piss Tollus off. Maybe once he realizes people are just being nice to him, because he's related to Tollus he starts to question things. Maybe he over hears the female cultist saying he's not as good at sex as he thinks. Maybe the men who he considers his friends are just suck ups. All this could motivate to him leave as he want something more genuine.
In the version where he's forced into the cult, maybe he uses his natural charisma to seduce women, and maybe he is an actual sex god, and he uses his charms to advance in the cult, until finally he finds an opportunity to leave. Maybe the cult is going to sacrifice him because they find him so insufferable, or they've created some new cuthllu type monster and that finally pushes him to risk it all in an escape.
And like you suggested maybe once he meets the champ he genuinely is concerned, but doesn't know what to do or who he can trust. This leads to him relying on the champion. I also think if he was better written so he came across as charming/funny it would do a lot to endear people to him. While I was never charmed by Zevran I will admit he has some of the funniest dialogue in DAO. Also I've never spared Loghain, but I absolutely love the interactions between the two.
But, the most basic thing Bubs could do to make Quin come across like less of a twat, is simply make Quin actually care about his own fucking life. Even if Quin was just helping the champion out simply trying to preserve his own life, I would like him more.
But these are my suggestions for a character I barely like, and only think about when people bring him up in this forum. It honestly feels like I put more thought into this character than the writer. Which if this is really a self insert, it is a pretty piss poor
power fantasy.