Making sweet, carbonated cider is a very popular idea. Unfortunately, it's one of the harder things to do because the yeast want to to eat all the sugar.
You want the yeast to eat some of the sugars to carb the bottles and then stop fermentation to retain some maple flavor. I've never done it, but in-bottle pasteurization sounds like your best solution to me. Read the sticky at the top of the cider forum. You want to heat the bottles, but nowhere near boiling. Pressure does increase substantially in the heated bottles, so be careful.
I wouldn't bother with priming solution. You can't get the yeast to eat only the priming solution. The yeast will eat some of the priming solution and some of the maple. After pasteurization, you will still have some maple and some priming solution. The remaining priming solution doesn't add anything other than sweetness and basically dilutes the maple flavor. If you skip the priming solution and only add maple, then you will only have maple remaining after pasteurization, which means more maple flavor.
A dozen vanilla beans sounds like a lot to me. I would think 2-3 would be plenty. You can add them directly to the secondary. I prefer to make my own extract. I slit the beans open and put them into a small container. Then, I add a little alcohol, such as vodka, rum, spiced rum, etc. The alcohol sanitizes the beans and extracts the flavors from the beans. The longer they soak, the more flavor you get. With the extract, I don't have to guess how much to add. I can add some, sample the cider, and add more if it needs it.