Too many Hungarians there. In Croatia you had 1% of Hungarians. Not so in Transylvania.
Yes, but unlike in Croatia, Hungarian would be an official language in Transylvania, so I don't think that would be an issue.
I think that could mitigate national issues, but not solve them.
Mitigating them would be enough, imo. Just like the Croats, Romanians in Transylvania would decrease to be a secondary issue compared to the other, unplacated minorities. In the meantime, this would also lead to said minorities losing quite a bit of weight. Who knows, maybe that in turn could make the Magyar leadership feel less threatened by them, thus the 1868 minority law would be observed more strictly and perhaps the sub-county level administrative divisions would be drawn/modified more fairly in the later decades.
Nationalisms were pretty stupid, you had examples of people going against the interests of their own country in the name of it - for example Croatian nationalists, who aimed for union with Serbia and when it happened, they mostly regretted it, one of them allegedly said that things were better in Austro-Hungary for them than in Yugoslavia.
And Romanians had their own country outside of Austria-Hungary and I think at least some of them will still gravitate towards it.
Sure, there's that, but even in the case of Croatia, A-H had to lose the Great War first for such movements to be able to prevail.
As long as there's peace and relative prosperity, your average man will not be willing to support or engage in any real separatist activities if they're not significantly inconvinienced in their daily life.
Religion can also play a role here. Just think about the case of the Southern East Prussia referendum post-WW1. Afaik, more than half of Transylvanian Romanians were Greek Catholics at the time, while Romania was overwhelmingly Orthodox. Sure, it might not appear to be as stark of a difference as Catholicism vs Protestantism, but it's a factor nonetheless.
On another note, in Romania itself, calls for unification with Transylvania could be somewhat lessened by such policy as well. It would be harder to claim that their brothers and sisters are being oppressed, albeit they could still point at the non-Transylvanian Romanians in Hungary, but the effect wouldn't be the same. It would be harder to rile up irrendentist sentiment.