Hire out as many household chores as you can afford housekeeping, lawn care etc. Put your best foot forward; be soft-spoken, courteous, well-mannered, chivalrous, and respectful. Not in endless discussions of temple marriage, not ever.
Which is the highest place in heaven aka celestial kingdom. Until you can believe someone else's beliefs are reasonable, you will not be able to treat them with the respect they deserve. In my view, baptism at 8 is just a variation on infant baptism. It can burn, and it can burn you hardcore. If she doesn't care that you aren't a member now, if your relationship goes on long enough, she is going to care eventually. What about the folks at church. I have a friend who identifies as bi-gendered and often feels most comfortable in femme. Trust your instincts and your spiritual promptings.
He also brought up issues with the church when we discussed religion so slowly he brought to light some disturbing facts and even though I would have denied it, it got to me. It sucks to admit, but I broke up with him after 6 months because I didn't think there was any point if we couldn't be together forever. This was hard for me because my faith is deeply rooted within me. I agree with what you and so many of the other replies have said: Listen with an open heart and curiosity. Joanna has written a good answer here. Sorry you had to go through this, but it's for the best if she's about to leave on a mission. I would not want my children raised Mormon which she seems intent on. And that fairly constant theme has some deep implications your girlfriend will have to face.
She is a returned missionary, and won't Marry you if you want to stay atheist. And, as I was writing the reply, above, those thoughts went through my head. Is there even any sex at all in this fantasy. It will be up to you, her man, to support the positive expectations. She likely believes that her time as a missionary was preparation for marriage. Mormons think when you die, based on if you were a good person cough cough atheists you go to spirit "paradise" where they will teach you the gospel.