Great points. I've thought about this a lot, and there was one Tweet that said Washington would likely take Robinson Beal is indeed off the board.
If the situation does in fact come up, and we take Beal with uncertainty about who Washington will take:
I would definitely be disappointed if we didn't end up with Robinson, but like you said, if we pick Beal, and Washington takes Robinson, I think the best option would be to keep Beal unless we're totally blown away by a trade. If Beal turns out to be the best option at shooting guard from the get go, then so be it. We aren't financially committed to Henderson, and I think he is humble enough to accept being the sixth man, and when he eventually tests free agency, he will find out if he is more valuable to another team than he is to us. The gap in talent between Beal and either MKG or Barnes is pretty significant, so I don't think it's worth acquiring the 24th pick if it means we end up with one of those two.
Edit to add: I'm sure this goes without saying, but that's only if we have the offer on the table from Cleveland. If they are non-committal, then take Robinson all the way.
In my opinion, you stay at two and take Robinson unless the front office sees him and Beal as identical players talent wise, but if we take the risk and take Beal, then Washington takes Robinson, we should roll with Beal.