U.S. citizens are visa exempt, so there is no TR visa in Canada for Americans. Normally, when they cross the border for a trip, they are allowed to stay for 6 months. Sometimes the passport gets stamped, but most of the time the officer doesn't stamp it, especially if they are coming by car because some people travel back and forth daily. If there is a stamp with nothing written or if there is no stamp, it is automatically 6 months. If they want to allow less than 6 months they will write a date at the bottom of the stamp.
If they are really worried that she might not go back the U.S., but they allow her to visit anyway, they will issue a "Visitor Record" (which I call a VR). It's a piece of paper which is folded up and stapled (or paper-clipped) to a page of the passport.
When I crossed the border with my partner, I told them he was planning to apply for PR and he wanted 6 months to visit. I had the application mostly completed, but I hadn't paid the fee. They didn't like the fact that I hadn't paid the fee yet (due to some confusion on my part), but they gave him 3 months on a VR, advising us to get the application in ASAP. They told us we were doing everything in the wrong order. The officer said we should apply first and then come for a visit. Possibly, that's what you should do, too.
We had planned to do a "conjugal partnership" application, in which we would provide evidence that we had already been in a marriage-like relationship for more than a year. After getting a lot of advice, it seemed a bit iffy to do that, even for a gay couple and it took forever to get the evidence together. At various times, I was so short of money that I thought I might have to go on welfare (which would have killed the application) and we had to keep updating the information and the forms. We eventually passed one year together and redid the forms as a common-law couple. (You won't have that problem because you are already married.)
A lot of things happened, but we finally sent the application off for an outland process in January. I just got sponsorship approval and we just got a 5th extension to the VR, so he's good till next January. We've been together for over 4 years now on visitor status, always thinking we are almost ready to sent the application, and have traveled to the U.S. to visit his family one weekend every year. There is usually very, very minimal hassle at the border when we return and they see the VR in his passport.
If you do an outland application, she has to be prepared to do an interview in the U.S. if it is required.
I have no idea if your step-kids would be able to get study permits. That is outside of what I know. How old are they? I suppose they can visit as your wife can, but going to school is a whole different problem.
