So the challenge is with a POD after 1400, prevent Castile's ascendancy in the Iberian Peninsula
After 1400? Nearly impossible.
At this date, Castile was the main peninsular power, and civil wars were more about inheritance than break-up.
Even if Portugal would inherit him, you'll have basically a different Spain given the demographical differences.
In fact, the best you could have would be, with such PoD, to have no union whatsoever with Portugal, Aragon or Navarre.
Then prevented to be the main maritime power it was (even if Galicia would still be an important point) and more focused on holding both Portugal and Aragon, you could have Castille as only one peninsular power and not the main one. (On the other hand, such configuration is likely to preserve the odds of a maintained Franco-Castillan alliance).
Super bonus points if you create a strong Navarre
Navarre, since the XIVth century, was more a buffer-state between Aragon, Castile and France than a real power of its own. No possibility of expansion (maritime or continental), three powerful neighbours, and kings tied up with their neighbours dynasties...
That's not the stuff you make strong countries with.
Ultimate bonus points if you can extend muslim rule in the south

(I mean Granada-esque rule, not the new Al-Andalus)
That's particularly unlikely : Grenada survived by the grace of being a tributary state of either Castile or Morocco, when not both. As soon Castile get its shit together, and manage to secure more pressing issues (as neighbours actually threatening), Grenada is roasted.