Would a stronger de la Cerda rebellion be the best chance of (re)separating the Kingdoms of Leon and Castile?
Could Leon later unify with Portugal or is Castile more likely?
Could Leon later unify with Portugal or is Castile more likely?
Well, try to write a little about what you are asking first. IIRC there were quite a lot of de la Cerdas.
Is Seville more geographically favoured by Castile? I understood it to be on the southern borders of Leon and New Castile/Toledo.Sadly I'm not too familiar with the event in question. But it is an interesting question nonetheless.
I suppose that the best case would be Ferdinand IV timely kicking the bucket (he is a kid in the Middle Ages after all) at the height of the double de la Cerda invasion. That way you end with a second partition of Castile and Leon with Seville going to Leon as they had agreed... for a while. Seville is too big, rich, and geographically favoured to be controlled from Castile to be left alone, so expect them to go to war against each other sooner or later.
Juan/John is no more Andalusian than his older brothers Ferdinand de la Cerda, Sancho IV, and Peter, but serving in Seville for so long did gain him loyalty there.Other option is to get Juan "el de Tarifa" to give up his claim on Seville earlier and concentrate on the north while he has Portuguese support (hard considering he's Andalusian but well, not impossible), then force a deal by which he becomes king of Galicia and maybe Leon at the cost of recognizing Ferdinand IV as king of Castile and maybe joining him on a war against Aragon and the other de la Cerdas.
Even though Castile managed it?Getting Galicia, Leon, Badajoz and Seville for long is akin to having a cake and eating it. Not going to happen.