I'd say that unless an Avernite candidate is simply absurdly good, we probably ought not to prioritise them. Syr is well respected, so I doubt we need to worry about political discord of any sort.
As you said, someone from Asgard might have the best chance of surviving on Avernus. However, while they'd be respected by Avernites, they might cause discord in other ways. Whereas Avernus seems to have a very collectivist, and broadly egalitarian society, that of Asgard seems to be far more stratified, with an essentially feudal nobility ruling over peasant serfs IIRC. That might potentially lead to some conflict and discontent.
A marriage with Vanaheim might potentially allow us to soothe relations with the conservative faction of the Adeptus Mechanicus that is so influential there. However, we already have ties owing to the fact that Admiral Freyr is Syr's Uncle IIRC, and the progressive faction is steadily gaining ground.
While a marriage to one of Garp's children might help to cement an alliance, I'm not really sure if it's necessary, or if it outweighs the benefits of an alliance with any of the other worlds. If we had more children, then I'd say we ought to be aiming for marriage ties with the rulers of every planet in the Imperial Trust, but at this point I think we should be going for the maximum possible benefits.
I still lean towards Muspelheim as our first choice, everything else being equal between the different candidates. Like us they have lots of resources, and they're also a key source of technology. Together we could exert a great deal of influence on the imperial trust, and if they were willing to share more of their secrets with us then we could benefit immeasurably.
In much the same way that Avernus has a population far smaller than many other worlds, yet nonetheless punches far above its weight in terms of importance, the same is also true of Muspelheim. It's an incredibly important world, and I think we should try to bind them to us if we have the opportunity.
