A 240k+ stage requires UCI dispensation, that is simply not happening for a major mountain stage. You also haven't explained why you think going around Stelvio would be better than going over it.Sure, you can argue that Stelvio is not in fact wasted, but don't call it a forced move when others complain about it.
Stages can be longer than 240 km.
If you could start the stage after Brocon from somewhere further north and/or west in Trentino than Fiera di Primiero, you could do Manerba - Brocon on stage 15, Livigno via Stelvio on stage 16, Livigno - Val Gardena on stage 17, Val Gardena - Padova on stage 18 and the remainder as planned. That is about the only option I see that doesn't require multiple stage hosts to be changed, and it would worsen the pacing (mainly in terms of backloading) significantly. This year was simply not a great opportunity to put Stelvio in a relevant spot, and as a result a year where putting Stelvio in an irrelevant spot was the most logical option. Like I said previously, it's the one annoying thing about this route that RCS had little control over.