User blog comment:Neetaku/Grisha Yeager is Alive/@comment-3547740-20150721021523

I suppose he could be alive. The fact that the legs were left over makes it less likely, though. It's possible Eren ate half of his dad, and then Grisha transformed inside of him in order to get out.

Personally, I think the author wanted it to be slightly ambiguous so there could be doubt as to wether Grosha is dead or not so he could bring him back of need be.

If Grisha's head got eaten and he regenerated from the legs, as you've stated (which I find unlikely since the nape of the neck is the regeneration point on the Titans and, theoretically, their human forms), then he'd of lost all his memories, so he'd be wandering around aimlessly.