1. No, you'll start losing the perceived bass as the system Q goes down.

2. If that is what is recommended then stick with it. It's a true bass reflex. No silly port there to add unwanted chuffing.

3. It's their modern XO so I would leave it alone. One thing I have found with the 604's is they require an EQ or at least the dual attenuator XO that came out with the 604-8H model, especially in smaller rooms.

4. Shelf bracing is the most effective and you can use cheap old MDF for them. Make sure you do not space them equally as you wish to have different free panel areas to reduce the amount of exciting that occurs at any given frequency.

5. Linoleum stick-on tiles and then sheep's wool. Don't be afraid to do all panels. Fiberglass works fine too. The stuff you show seems rather expensive considering what it's doing.

Good luck with the project.