I think that no-expulsion is an easier POD than no-christian conquest. Not because it's imposibile to keep muslim states alive in Iberia, but because we have to change less things.
That said, in the debate about the christian conquest we are talking mostly about military posibilities, but this overlooks that the proccess is not only a military one, but also (and maybe most important) a political one. It would be an error to understand medieval Iberia as a clear divide between a christian and a muslim bloc of sorts, and things were really more complex. Think that Iberia is a peninsula, is not an island, and the Pyrinees and the straights allow to intense relations with the outside world, but not as intense as it would be in a geographic continium. In this defined geographic space, both christian and muslims played often a common political game, where religion could or couldn't be a factor, depending on the coyuntural interests. Even you have figures like
Abd al-Rhaman Sanchuelo, whose mother was a christian royal, or
Sancho Alfónsez, heir of Alfonso VI of León, whose mother was a muslim royal. What probably played more against the iberian muslim states was their political atomization, as is often said, but I think that mostly because that made them politically waeker in the game of alliances, vassaliages and influences rather than because it made them military weaker. In this sense, the protetction and intervention of Maghrebian empires created tenssions with the andalusians, but no more than the intervention of european forces created tensions with the iberian christians. The epitome of all this is perhaps the Battle of Las Navas de Tolosa, where both armies marched heavily divided internally in thoses lines. The christians crussaders were invited to go home before the battle, and the andalusian units departed the battlefield soon after the first clashes, because apart of the mlitary question, their respective allies where unable to understand the political game and the conventions forged during centuries of cohabitation. Meanwhile, while the castilians, portuguese, navarrese and aragonese had their gaze in Sierra Morena, the leonese were profiting the opportunity to sieze castilian fortress in the common frontier.
But as said, I think that is easier, regarding the search of a POD, to avoid the explusions and (systematic) conversions of muslims. The legal basis to get this already existed in OTL, and Alfonso X's Partidas (a legal corps with a more liberal spirit than anything made in this neck of the woods until the 1800's) had provisions on that sense, and they are they didn't come out of the blue, but after the refinement of an existing juridical tradition. The policies followed against muslims and jews later in Sapin are not the product of a fanatical, blind hate against religious diversity ingrained in the spanish souls (the episodes of hate existed and caused problems, but they are not systematic), they are actually the product of something even worse. It's a cold modernist push for a royaly centralized and homogeneized (in a pre-nationalist sense) state, but even that push could have been managed in a different way.
Here, the aftermath of Las Navas is again a key point.A problem faced by the castilian crown in the subsequent quick conquests was its inablity to manage those news territories, and this law had deep political consequences. The new territories were left to the control of the big nobility, unbalancing the political picture (less overall power for the cities and free paesants, less opportunity for the proto-liberal line) and an inomming political struggle between the crown and the nobles that reachs its definitive points, for what matters here, with Ferdinand and Isabella and with Felipe III. So, I think these are the lines to think about a POD to get what the OP's wants.