1. For 3 season use, you could just use the cell foam panels. But I, like you, would be tempted to put something else over it as well. I would probably go for a thin layer of closed cell foam, with safe insulation in the less flat areas.
For winter use, use thinker closed cell foam, and fill more of the gaps.
2. 6mm is fine. I would use something thicker, to make it warmer in the extremes of spring and summer. Depends on your available headroom inside. If you’ve got the space, and a little more budget, go a little thicker.
3. All sounds good.
4. I haven’t, and have not had a problem, despite all weather conditions, and 2 people.