Voidsingers are best for any civ that can generate a good amount of faith. You want to go for monumentality classical or medieval pushes anyways with Poland, and the Obelisk just helps with that. If you build an obelisk as the first thing in every expand, and settle cities on turn 30, 40, 50, 60, 70 (which is not a very aggressive estimate) it equates to 600 faith by turn 80, which is around 2 settlers.
Moreover, the Medieval era promotion of the Voidsingers is just insanely strong. It's pretty easy as a faith civ to get to 200+ faith production by the medieval era.
I think the Owls don't have a huge amount of synergy with Jadwiga. Banks are pretty crappy buildings and they don't get that much better as Gilded Vaults, unless you have obscene Commerce Hub adjacencies which Jadwiga doesn't have much support for. Moreover, the economic policy slot is less useful for Poland than for other civs, as Poland already has a Military to Wildcard conversion. The only real synergy I see is that Jadwiga likes international trade routes a bit more so can use the city state trade route envoy ability well.
Hermetic Order is just weak. There's basically no civ that isn't better served by picking a different society.