I don't think Hull are looking to lose an FA Cup Final to get a pass into Europe. If they happen to beat Everton before then that will help them, yes, if it helps Arsenal and they obviously know that but they weren't going to not try any less against us - or even not try to win the Cup Final. Their advantage in knowing if we finish behind Arsenal (in 5th) puts them in Europe so they can use that advantage - it's a perfectly fair one (for them) because to take advantage they have to beat us - not lose!! They have earned that right to the advantage, such as it is but they have to win a game to use it - that's what football teams try to do!!
Normally teams rest players before the final - or at the least they are wary of picking up an injury. So it might be an incentive to beat us but so be it.
If Arsenal happen to finish 4th then they go into the final knowing they are already in Europe. End of.