Really? I’d always thought Isabella was the driving force of the Inquisition. If so, that’d have some big consequences culturally in Spain that I’m definitely not equipped to discuss tbh
The Castilian Inquisition was rather "lax" until Isabel "reformed" it. Ironically, she reformed it along the lines of the Aragonese model.
Why is Isabella out of question?
Because she didn't understand the Aragonese government. She was regarded as a foreigner. She wasn't just the queen-mother, she was a queen-regnant with her own kingdom, so where there was "equality" while Fernando was alive, here, Aragon would be directed to Castilian interests (like making peace with France).
Absolutely nobody will allow a bastard to rule in the name of the king, imagine the precedent it will set.
By that reasoning then the entire Trastamara dynasty should never have come to power. Since they were
also bastards who displaced the rightful king. That being said, the Aragonese were
more than happy to accept Alonso as regent due to Juana la Loca's "insanity" here, why wouldn't they be willing to accept it here? Besides, an added bonus is the fact that Isabel la Catolica
loathed Alonso (actually, all her husband's bastards) and he
was actually a capable politician (IIRC, even Juana la Loca and Karl V were wary of Alonso's "political powers")