I have a ton of normal garden tomatoes -- would using them make this too wet?

It will also depend on the tomato variety. Some are juicier than others.

In any case I would be inclined to seed the tomatoes as the seeds are often bitter.

Again this is will depend a lot on the specific variety(ies) growing in your garden and how the plants responded this particular growing season.

It's easier to introduce some water into a recipe like this to rehydrate it toward the end rather than try to evaporate liquid.

Use your best judgment.

It might, but you can always compensate by roasting the vegetables a little longer.