Spain without Catalunya is still much more than Castilla-León.
Anyway, as someone else said, the loss of cuba and Filipinas actually was a boost for spanish economy, ending with a constant drain for the national budget etc.
On the other hand, the theory os spanish backwardeness needs a lot of nuances. It has been used inside and outside spanish hisrtoriography, for different reasons, but in recent years it has been pretty much debunked. Certainly Spain was not in par with the top industrial powers, but she had a very typical european, unless we limit europe to a handfull of countries, proccess of industrialization since the last third of the 19th century, as her political proccess was not such an exception neither.
Eeeh, no, rather the opposite. The Bourbons have been expelled twice by the people, and hated by large chunks of the populations since their arrival, including a terrible civil war because their proclamation as kings or crowds threatening with storming the royal palace in times of Charles III (and the king flying Madrid) or armed guerrillas against Ferdinand VI. They have been restored, however, despite popular opinion twice, the first one because it seemed there was not other option (1871) and the second one because a brutal dictator said so (1975)
But yes, the OP seems very unlikely and totally ouit of touch wirth spanish historical dynamics of the time (with respect). Had the monarchy fallen, a republic or a dictatorship are the likely outcomes, nobody is going to make a commoner anew king, that has not historical precedent in spanish history, nor makes sense in the context of the time. Noiw, you need good reason to make this revolution possible, and depending on the created political conditions we can think wether it will lead to a republic or a dictatorship.
Even more difficult is to make Catalonia split in the turn of the century, when a very tiny proportion of catalans supported independence (geez, they are not a majority even now) and when the open goal of catalan elites was to become hegemonic inside Spain, not to break it.