Well, demodex are meant to live in our pores so I guess when you see them crawling on your face, they are just moving from one point to another.
Every studies say they don't have any kind of anus, so when they die they release all the shit they were carrying. But I guess you knew that already. Anyway the issue wouldn't be when they die on your skin, but inside your skin, your pores.
Concerning ivermectin effectiveness, I suppose oral form on its own is not enough, and topical forms are usually more effective when the disease is not about the whole body (at least for bacteria).
We all would need a skin biopsy to make sure demodex is the right culprit, but from what we know, it all makes sense if it is it.