I am into getting fisted and playing with rather large toys. One of the toys I play with regularly is 16" long, and I am able to take about 12" of it. (There is only about 2" of it that I cannot take, not including the 2" of balls on the bottom.) No, it does not go directly to the belly button.
Normally, you do not feel a guy actually cumming inside you. You might feel his cock throbbing as he shoots. But I have noticed after getting fisted, my insides are well used and I am able to feel when his warm cum hits my insides. But that doesn't happen often.