I think a big penis does make a difference when it comes to insemination. It can put the semen deeper than a smaller penis ever could and that does matter
Happens. It's funny I have had a few recipients who for some reason wanted to try a few men at a time. I will say it puts me in high gear to cum deeper and harder than anyone else. Competitive drive for the egg.
I've had a few of those who I inseminated a few times but ultimately another guy knocked her up. Sometimes I think there's just a mismatch that makes it harder Rh type or something. But yeah always worth a try.