Ok - that sounds to me that the person you spoke to is not closing the door to reaccommodating you on an other airline if need be, but are hoping that in priority they can rebook you on U2 and don't want to clear the rebooking on others now (which obviously costs them money) that they then could not "take away" from you and find that they have a seat for you on their own planes after all.

It's a bit frustrating as you want the certainty, but I would say not entirely unreasonable. And I guess the question if for you to decide how long you are happy to wait before being sure. Given that there are 3 weeks before your flight, I'd say that waiting 24 hours should be ok, but if there is still nothing then, you might try and negotiate a deadline ("ok, I'm happy for you to keep trying till the end of the week but if there is still no U2 solution by Sunday, I want you to agree to then rebook me on another airline" or something like that).

Their bit on since they have offered you the refund or data change that matters more is nonsense. The regulation is clear that it is your choice to select any one of the different options the regulation foresees that works for you best. Often, airline will try the "oh but we offered you a full refund" line to sound like they are being generous, but it is not for them to decide if you want to be refunded, travel on a different date, or travel at the next available opportunity, it is for you alone to choose depending on what works for them and they are not in a position to limit your options among those.