muse, most of what you have written sounds a lot like many people's first experiences with poly. There are some who are lucky enough to know from an early age that what they wanted wasn't monogamy, but the rest of us kind of went along wit it because we felt that there was no other way.
So the stuff you are talking about is quite, quite normal. The only "red flag" is the part about his other lady not accepting you in his life. That is going to cause massive troubles for you while trying to work this out, because you aren't really all working for the same set of goals.
If she insists on keeping this viewpoint, then poly between the three of you just plain isn't going to work.
"Listen, or your tongue will make you deaf." - Native American Proverb