OTL Portugal DID continue a reconquista towards Morocco by conquering certain coastal settlements. Though it did not continue further than that, it did happen. AFAIC, Spain has potentially more resources to get something. I wrote about it earlier in 2019. But in this case, avoid Spain ruling the Lowlands and let them just keep their Italian gains. They need the part North of the Rif Mountains, the area they got in the 19th century. Populate the area with Christian Iberians and you have secure North African base. The danger is that Spain now has control over going in and out to the Mediterranean. Which means, any power willing to fight wars with Spain will concentrate on there and make formal alliance in reconquest of the region. Nothing impossible but not something that will make it go smooth either. From the Rif region, future expansion can go. This will however require a lot of time and resources. And the Moroccan population is not really willingly to surrender either because of the religious policy of the Spanish until the 18th century. So a substantial population of Moroccans will remain as Muslims.
I wouldn't be so sure about the last parts.
Unlike Nasrid Granada, (Northern) Morocco, by that point, hadn't had any Christian neighbours for almost a millennium. Berbers aren't exactly known for their piety and how easily they convert, hell, the Almoravids and Almohads had embraced asceticism and mystics in Morocco and Al-Andalus, despite the rigorous Maliki 'ulama proclaiming otherwise in Seville. I don't think you could convert the population that is known for its pride and resilience that easily, especially not by introducing Iberians into that mix. And if we throw in that holy office of the Spanish Inquisition... a perfect recipe for a disaster. This TL would probably even further develop some more extreme racist tendencies than the Spanish did towards the Moriscos (who were already Christian!) before.
Another thing to note is that this Kingdom of Mauritania is surrounded by Muslim Kingdoms who could definitely count on local support.
Last, but not least, there's nothing economically viable enough to support such an early expensive colonization effort which ought to be protected by the Moroccan sultanate, some Berber raiders from within and without, a rebellious local population, Algerian imamates and sultanates to the East who might sooner or later call in the Ottomans. The Rif was not as strategically important as it was by the dawn of mechanized warfare and outside of some iron and coal mines near Oujda, Figuig and Nador that were only first opened in the 20th century, there's not much of value that couldn't be produced in Spain or the New World.
I'd therefore argue that Spain, despite its wealth, most likely won't stay around there for long. The Spanish must be damn lucky to get around a population they can't just throw out and neighbours perfectly capable of challenging their presence in Africa for more than half a century. More than two centuries might even be ASB, if other Muslim nations and the locals don't suddenly get snapped away into irrelevance.
That's at least how I see it. North Morocco isn't just Melilla and Ceuta after all, it's a large and difficult-to-control mountainous area perfectly suitable for nasty guerilla warfare in the event of hostile occupation. Battle of Annual, anyone?
