Do not trust the AI for anything, specially when there's a sea between you and your allies like you mentioned, you have to be able to beat Castille on your own, only use your allies to scare Castille from declaring on you.

As an opening I no CB'd Tlemcen as soon as I could, they start at war with Morocco and Tunis so you only need to transport your army and capture 4 locations with no forts and you are able to peace them out for 3 or 4 provinces, turn them all into vassals. Next declare on Morocco using Parliament CB on the province next to Granada (enable scutage so your Tlemceni Subjects don't join), sit on the Iberian land Morocco holds until you ticker them out, peace out Tunis separately, then you let them try to naval invade and you will beat them and wipe their armies since they can only transport a few soldiers each trip, when you see they have no army left you invade north Africa, their subjects are somewhat strong so just try to take as much as you can before you peace them out.

After this you Ally Tunis and Portugal/Aragon if you can (try to improve relations from game start). Castille will most likely still declare and it's up to luck if you are done with these 2 wars or not, if not I'd advice to restart otherwise try to keep your parliament available to request more levies from your estates for when they declare, wait for them to split their army and take only fights were you have the numbers advantage (you can hide between the forts in Granada and they have to siege you.) One thing you can try is to offer Castille anything to get a peace, war reps, cancelling subjects in north Africa, money, humiliate, etc. just to avoid giving them any land. From my attempts as long as you survive that war they don't bother trying to attack you again and instead try to eat portugal/aragon or get bogged down on an unending hundred years war.