Philip II ended up marrying Anna Anna of Austria, his niece, as his 4th wife. But she was close to marrying her cousin instead, except he went crazy. (This could only happen to a Hapsburg.)
Suppose Don Carlos doesn't fall down the stairs and need the hole drilled into his head to let out the excess fluid or whatever. Just how much of Don Carlos' problems were after the fall, anyway?
Let's say he has some mental problems but isn't off the deep end. Would he have had any less successthan Philip II at siring offspring, or more, do you think? She had several possible heirs with Philip II; it's just that all but Philip III died very young. Would Philip II marry again? My guess is yes, but who - another Austrian perhaps? My hunch is that his policies would not change that much - but there would be a very different successor or perhaps none at all.