Hmm, I don't think I've ever had any problems closing a minor rift within the time limits with 2 characters. I don't think I've failed to close a major rift with 2 characters either, but I can't finish the bonus in time. Last night I tried to close a major rift with 3 characters for the first time and I almost made the time limit. It seems doable.

I haven't tried many expert rifts at all yet. I'm leveling up some new characters at the moment.

I've tried to sell a few (6-7) planar dust so far, and they were all sold for around 5p buyout.