The xp rewards are higher if you finish faster so that there's no motive to intentionally drag things out. I can tighten up the min/max range a bit (not lowering the max, probably), if it's an issue but in general the usefulness of the champion will be proportionate to how well you do with it. A lvl 45 battleship is not just a gimme, you have to work for it.
Though the last version did make it easier, in terms of making the scenarios easier.
Also, gaining some early levels off the AI is definitely not a bad thing to do; the scenario XP rewards scale up as you're higher in level but the normal-space rewards don't (so there's not much motive to kill guard posts with them past the very early part).
As far as those only updating on official versions, we are hoping to do 6.1 a month or two after 6.0, so we can update to the newer unity version, so the wait won't be so long as this time. But yea, there's tradeoffs both ways to playing betas vs only the releases.