Can puff pastry be used instead of dough?

Yes, for sure - I have made very similar things with puff pastry. This is the kind of dish you can riff on endlessly: swap out the pastry, swap out the vegetables, swap out the cheese. You just have to think about when and how to layer what so that whatever is on top doesn’t burn after 35, 40 min in the oven.

I would think the answer is "Yes" to all three of your questions, though I'd slice the Brusslies, as they generally take longer to cook than chard or kale. Sounds delicious! Let us know how it turns out, please. ;o)