I can only give you my own experience and fwiw I am currently pg with DC 3 and feeding DD2 (11 mo).
I never had any pain, but it did feel really weird for a while when I was pg with DD2. But the weirdness went away a little right after DD2 was born. Actually, if you end up suffering from engorgement, toddler bfing can be the best painkiller ever. Again, this is my experience, no guarantees.
DD1 did not ask for more feeds when baby arrived, in fact, that was partly when she started to really tail off. I tandem fed both of them which for my DD1 averted some of the jealousy.
BFing does cause your uterus to contract, I forget why exactly, but I remember reading about it. It didn't really get worse at all for me. I just practiced my breathing
.
As to your last question, DD2 was born at about 39 weeks so I did indeed carry to term.
Also, if it is painful, it's ok to tell your DS 'No, not now, that hurts' and try again later if you want to. I did that.