So for starters, I don't think your first point has anything to do with politics in any sense, but I agree it's odd for Astarion to have had a negative reaction. It seems like something he'd approve of. Shadowheart though, she does have a consistent tendency to prefer helping out the downtrodden when it doesn't hinder your own efforts, which wouldn't have happened in this situation.
Second, the point of it is actually laid out later I believe. It's part of Gortash's plan, using the refugees as an immediate external threat alongside the coming army to stir up fear and discontent tht he can use tofurther solidify his hold on the city. Also, realistically that's something that would happen. A vicious army is on the march, people are going to flee it and go to the nearest place that seems secure, and historically the populations of those places get up in arms about the influce ofoutsiders. I don't think the writers were trying to specifically reflect any particular refugee crisis, it's just that a lot of refugee crises play out rather similarly. This is no more inherently political than the Lae'zel's storyline about the potential Githyanki schism caused by a coniving, selfish leader who abuses her people and the meditation on faith and loyalty that comes with it.
