Did Castle get married on the set of Jane the Virgin’s telenovela? And did brunette alt-Alexis remind you in the slightest of Camilla Luddington? Also, are we to infer that in the AU, The Wire’s Detective McNulty worked for the NYPD and enlisted Rick to solve the copycat murders?
I'm only qualified to give my own answers/interpretations to two of those questions.....
'Castle' takes place in an alternate TV dimension from Earth Prime-Time because of the black mayor who used his influence to get his friend Rick Castle "observer" status with Detective Kate Beckett's team at the 12th Precinct. Because Mike Bloomberg was the real NYC mayor at the time and has appeared as himself in several Toobworld-based shows, 'Castle' had to go.
But that doesn't prevent other TV shows from having counterparts in that alternate world as well. And even if the characters from 'Jane The Virgin' don't exist in Castle's world, what's to stop that telenovela from being broadcast there as well?
Rick Castle has a lot of influential friends in all strata of society, including the television business. So it would have been no problem for him to get his wedding "staged" on the set of that show.
Detective McNulty is definitely in the world of Earth Prime-Time and would have been working for the police department in Baltimore. But he could have a doppelganger in Castle's dimension as well. It's not so unbelievable that a cop from Baltimore should be working for the NYPD.
Just ask John Munch!