Essentially what Constantine said, after some convincing that the Peninsular War might not have been a lost cause, continuing the illusion of Spanish self-rule would've kept Spain going in the long haul, especially in the face of the dithering of Carlos IV and the untrustworthiness of Fernando VII in Nappy's eyes. Francisco de Paula is the best bet because the other two options are hostile to Nappy's interests (Carlos, Duke of Molina/Carlos V was a staunch absolutist and would probably join the coalition in a heartbeat, while Antonio Pascual disliked Godoy and supported Fernando, so probably in a similar situation to letting Fernando keep the throne) plus Nappy might find controlling Spain by the reigns of a fourteen-year old boy more logical than putting his weak-willed brother there.
Still, dunno how feasible marrying Joseph's daughters would be, considering he's 14 at the start of the OTL Peninsular Wars, whereas Joseph's daughters were half his age (Zenaide being 6 going on 7 and Charlotte being 5.)