One factor as well, regarding the Thrall question, I am pretty sure that all of the other houses practice Serfdom. Now serfdom is not as bad as slaves/thralls in many ways, you cannot just buy or sell people, etc, the degree of labour they owe is likely very sharply constrained through law and custom, but they are still tied to the land and unlike with thralls it is generational and inherited.
So there is probably a degree of not wanting to start throwing stones when you live in a glass house.
Added to that it is not as if the Crown can really make one of the great houses do something if they do not want to, the Compact is very much an alliance of necessity and convenience against the far more numerous Abandoned, who outnumber the Compact about two to one in overall population.
Added to that just how would people invade Thrax? Redrain is entirely land locked, Valardin is literally the other side of the continent, you would basically be limited to the Crown, Grayson by extension then maybe some of the Lyceum if you could get them to be interested. Given Thrax would be defending and on islands, with sieges of castles being drawn out and hideously expensive things, I do not find it surprising that nobody has ever done it.
Remember that Redrain does not even follow the same religion as the rest of the Compact, despite wars fought in the past over that very issue.