I know what you mean, Yseult, but still, I don't think Spain would recognise a private renunciation done within the Royal Family. If Pilar and Margarita wished to push their rights forward in the eventuality of all Juan Carlos's descendants dying, I'm sure the laws of Spain would give them coverage...
About Luis Alfonso, that's a difficult issue. I've always thought don Jaime's renunciation was not legal (as it wasn't approved by the Cortes), but he then married semi-morganatically and don Alfonso's marriage was nothing else but morganatic. So, as the Spanish monarchy we have now is Franco's creation, old rules should not apply to the monarchy, so whether Jaime's renunciation was valid or not, Juan Carlos is true king of Spain (obviously). But, if all the line descending from don Juan should run out (impossible, I believe)... yeah, I think Luis Alfonso would have a chance.