1) Yes, plus a legrest.

2) While it's around, Flagship First is in theory a better hard and soft product than Flagship Business, but it is only available on the 77W and A321T. The 772 has Flagship Business, Premium Economy, and Main Cabin. Not sure why you're seeing only two cabins. I'd say Premium Economy is still materially worse than business class 15 years ago. The latter was angled flat seats (on AA at least), whereas the former is, as you say, a slightly better domestic first class seat (still worse than the previous generation of business class recliners from the 1990s and 2000s, which had at least 10-15 inches more pitch than Premium Economy today).

3) No difference on AA metal, I'd venture it's just a display glitch. Any itinerary with DFW-NRT in the front cabin will grant you access to Flagship Lounges and Admirals Clubs at any airport along the way, as well as the JAL business lounge at NRT if connecting onwards.

4) I think it's worth it, but be prepared to not clear. It will not cover the JAL segment.

5) D and H for a couple.