First, you need to stop worrying about Donna's interactions with Richard, New Neighbor, or anyone else who isn't you. They're adults, let them handle it.
My impression is that, while Donna may indeed be very passionate about her hobbyhorses, she's also quite lonely. I think you need to stop hearing her rants as trying to give you information, and start hearing them as "I need to have a contact with another human and this is all I can think of to talk about, I need validation, I need need need." She isn't trashing your different choices, I don't think---she isn't thinking about you at all. She just wants an ear.
Which is not to say it has to be yours. When there's other people in the group, I think you have to let her talk, but when it's just the two of you, you can hold up your hand and say "hey, babe, I know---you've told me before --- I know you know, you know I know, you know?" 
or something equally light. Ask her if she belongs to any forums or groups on these topics. Ask her if she goes to meetings or is otherwise involved with people who share her interests. And then find something else you two can discuss.
(I hope she isn't equally "passionate" about current events, movies, the weather, what-are-you-doing-for-vacation, etc etc.... 
)
Calypso, you make a very good point. Donna moved here from faraway. The move was initiated by her husband's company and they really didn't have a choice (since they are a single-income family). She doesn't have a single family member nearby and almost no support network. From other conversations with her, I know that:
1) Our small conservative little town was not her first choice. She hates that it is a 10 minute drive to get to a grocery store and about a 40+ minute drive to get to a health food store. Heck, EVERYTHING is a bit of a drive. I love it here, but it is not for everyone!
2) She and her husband went through a very rough time and almost divorced. They worked it out, but I suspect she only stayed for the sake of her three kids. Her youngest daughter (Kitten's friend) did not handle the fighting well at all. The move away from everything she had ever known only created more stress. Donna has been dealing with the fallout and has had her hands full with this child. I think she actually enjoys it when her youngest spends the day at my house playing with Kitten because it gives her a break
3) Her oldest daughter was a premature baby and has serious health issues. One of which is something similar to ceciliac disease (thus causing Donna to have to research special diets, etc). She also has severe learning disabilities and she had a bad experience with the public schools where she was living when OD started school (thus why she started home schooling her child and decided to do it with the younger kids too).
4) Her husband travels all the time (like 80%) so she is often left alone in their home with three kids. Of course, she gets no break because they are home-schooled and involved in extracurricular activities up to 30 hours per week each.
I am trying to be understanding and believe it or not, Donna and I do have many things in common. We are both liberal thinkers in a conservative environment. We have similar child raising philosophies (we're not helicopter parents like so many of my friends
), and other positive stuff. I would like to keep our friendship...however, I'm really tired of these two topics dominating EVERY conversation. I like your suggestions and will try that next time. I will probably see her this weekend, so I'll try to post an update if I have one.
Thanks again!
P.S. - Funny you should mention her passions...she also has VERY strong opinions about the TSA...but that topic only came up when I mentioned we were flying for our vacation.
