There is anal play and anal intercourse, preferably in that order.
The anus and sphincters are designed to pass matter outward, not accept it going inward. Please read this article:
"Playing Ball" and How To Tips for Prostate Massage
This is also an excellent tutorial for people interested in and concerned about the how-to of Anal stimulation as well as play.
The play helps condition these body parts in order to make the passage of larger items (dildo or other toy, penis) easier. Using lots and lots of lubrication and proceeding slowly using gentle pressure are keys.
The membranes are thinner than those of the vagina yet should not stretch permanently out of shape unless a fist or other large object are used regularly.
I do not have answers for your other questions; however, I also believe that the anus is not the place for a penis, covered and protected or not.