who in any case has two Anglican daughters and seems unlikely to produce a son
I never understood this "mentality" of the people in 1688, that JFES was a "changeling" because James
couldn't father kids anymore. He and Katherine Sedley had had a kid as recently as '85/86 IIRC, and Mary of Modena was only 30. Henrietta Maria had been older than that when Minette was born and despite the whole ECW going on, nobody ever alleged Minette was a changeling.
William would then claim the power. And given England's naval and army strengths under the Stuarts, William would most likely get his way.
Would he? Or would Monmouth be able to garner more support locally. True Charles II is dead with no chance of "posthumously legitimating" his eldest, but a dead Charles also can't refute the allegation that Jem Monmouth
isn't legitimate. Not to mention the fact that Jem was able to have support for his OTL rebellion would suggest that he
does have a chance. After all, Will
deliberately screwed Monmouth over to get a rival out of the way (making noises about supporting Jem then said support not materializing; not to mention thoughtfully supplying a list of Monmouth's co-conspirators in the Hague to his uncle/father-in-law). Which would suggest that at least
some foreign governments certainly saw Jem as an alternative.
That said, Jem, while a good commander, was also not the brightest crayon in the box politically (that was his wife, FWIG), so it could be interesting to see how it shakes out. OTL, William had an invading army
and local support (plus a rival who was Catholic). Here, William has an invading army, presumably very little local support, and a rival who's a Protestant. That Jem's a bastard is awkward, but not insurmountable. William has no kids, so when he dies, who gets the throne? Jem's got several by this point (IIRC). Not to mention that in 1688, William had had time to "prepare", in 1683, the Plot would've been sudden, so little to no prep. Equally, in 1688, William had foreign support (including from the pope and the emperor), would he have that in 1683?