I don't see why not, if Frederic holds by the Caltabellotta Peace. That said, the risk of having this one being broken is still important, and I'm not sure the project of union would last giving Bonifacius was dead.
I'm not sure to understand. You mean the political reason? Well, Angevines were Capetians, and depended a bit (for their mediterranean policies) of the elder Capetians support.
This union could have cemented a bit more the Treaty of Caltabellotta, meaning a possible lasting peace if not Sicily turning back to Angevines according the treaty's clauses.
(It could have well been a pressure on Charles II, giving his support for Bonifacius at this time, but I don't find a year for this union project).
If they still die ITTL, probably.
As such, probably. But tensions grew enough and the Capetian/Plantagenet conflicts were already an established situation at this point. So, you'd probably end sooner or later with another war, which may be harsher than the precedents.
Maybe. It would probably follow the "Plantagenets kings are bad vassals, many of their own vassals complain to me, their ultimate suzerain. As he doesn't wants to humiliate himself, I've confiscate Gascony and go to war against him. What a shame."