A straight-laced, returned sister missionary, raised in the cult, and in family of similarly entrenched cult members, will not likely remain happy with a non-member. She's told me the church is one of the most important things to her, but from what I've researched already there's no way I would subject my future children to this culture. With all of this said, I wish I would have found your blog earlier. A lot of what you say about Support was what I expected to find in residency.
I've been married to a cardiologist for 30 years. That settles it for me. I am a career military man's wife. Of course, your parents will care most.
Keep the relationship casual. The fact she's dating you while you aren't a Mormon is at least a sign of hope. They seriously have movies about it. We also live in a foreign country which is another element of difficulty. Hire out as many household chores as you can afford housekeeping, lawn care etc.
If she expresses interest in converting, be prepared to talk about your beliefs, and help her find ways to learn about and follow them. Personally I have a rule that I will not enter a relationship with a believing Mormon. There is a difference between commitment and time together. They know that they are the best.