How do I thicken pumpkin pie filling?

If you haven't baked it yet and you're worried that it won't set add an extra egg yolk and stick it in the oven.
But.
By boiling the maple syrup you're cutting down the amount of liquid in the recipe (as well as concentrating the flavour), since a lot of the water has already been removed in order to make it into a syrup I wouldn't worry too much. It's quite likely that if you baked it as is your pie would turn out just fine, although it might need a little extra time in the oven.

That's a tricky one, since you're thickening and almost candying the maple syrup. I make a vegan pumpkin pie without eggs, using just cornstarch to firm up the custard. That may work for you--dissolve the cornstarch in a small amount of liquid and whisk it in before baking. Good luck!