The first scene you described reminds me of a story that I think was called something like
The Western Bond. I might be wrong about that because I'm having trouble finding it, myself.
It's been a long time since I read it, but I definitely remember Kagome and Sesshoumaru being mated, and one of the other lords offers Sesshoumaru his own daughter as a concubine. I think Kagome was also immortal before the mating, due to the Shikon jewel's connection to her, and had known Sesshoumaru's father long before Sesshoumaru was born, but I could be wrong about that. Also, there was another lord that I think Kagome gets involved with while pregnant with Sesshoumaru's child, but I'm not sure what happened after that point -- the story had only been updated to that point before I lost track of it.
Anyways, I'll keep searching for it and put up a link if I find it.
Edit: I just found a posting on another topic in the forum about how the author pulled the story I am talking about from the web in order to rework it into an original fiction.
Kagome gets cheated on