First of all, we have no idea how capable Senya is as a duelist so Vaylin losing to her doesn't really say much about hers(I bet they are both pretty good actually). Also, Ant makes a good point as Vaylin seems to prefer abusing her force powers and her fight with her mother seemed like a very personal instance to me. In due time we will have a clearer understanding of her level of skill(hopefully Senya or Vaylin will get new feats) and Jaina vs Vaylin will have a more solid conclusion but I think even as of now, Vaylin wins due to better application of the force.
HoT/The Outlander's skill with the blade shouldn't be underestimated and Arcann was his slight superior so closing the gap wouldn't be a quick victory for Caedus. Arcann can definitely hold his own in the force against Caedus longer than Jaina can hold hers against Vaylin.
Since this is Valkorion with his KOTFE showings(they still don't represent his full power), I believe the fight would be very similar to the force fights between Yoda and Sidious. Since they are very close, it should again take longer than Vaylin's fight.
But lets say Luke manages to kill Valkorion. He can still come back and enter Vaylin or Arkann's body and unleash his full power there yet again.
I don't think team Skywalker can win a majority here tbh.