Hello!

In the past year, I've switched over from flying other airlines to mostly AA due to change of locations I need to get to. I have not taken an international AA flight in about 15-20 years, and I was a young teenager at that time so I am guessing a lot has changed. Obviously the international landscape has changed a lot with the last leg being flown as codeshares where as I remember as a kid, we could get a ton of places across the world on AA and Northwest (and later Delta) metal.

I do travel a lot (Platinum Status, which I'm not sure I benefit from a lot) so my questions are mostly aimed at understanding the premium international classes. Please correct my misunderstandings and help guide my confusion as below:

1) Premium Economy seems to be a domestic first class seat that is stuck in a wide body--some upgraded food options and a comfy chair with pillow and blanket.
2) Flagship Business / First are essentially the same product with different names? I'm looking at a 772 flight from DFW to NRT which seems to be a 2-class airplane--Premium Economy seems to be a part of the main cabin, but the Premium Economy is what the old "business class" was on a plane with 3-class international service 15-ish years ago was?
3) What the heck is the difference between Flagship Business and International Business? If I book a flight from my Origin airport through DFW, get on AA 61 to NRT, and end there. It shows my option in the top-tier cabin as Flagship Business. Same flight from Origin-->DFW-->NRT-->TPE with the last segment on Japan Air metal, and now that same seat on the same AA61 flight is just regular Business. Looks like same onboard amenities (which makes sense given that it's the same physical space), but the difference is that Flagship Business passengers can access the Flagship Business lounge in DFW and the Admirals Club at the origin airport, whereas international First can only access Admiral Club at DFW and have no access at Origin airport? Guessing it's a "penalty" for not flying AA metal the whole way, but in exchanges, I'm guessing you get access to the Japan Airlines Lounges at NRT before the NRT-->TPE Flight
4) I'm reading conflicting reports on how to upgrade to Business/Flagship Business with points. On the website, it says 25,000 points plus $350. However, I'm reading that paying that amount only gets you on the list (there are no free upgrades for status members on international legs, but there are for the domestic legs). If that's the case, is it worth almost $650 in points and cash to just "get on the list?" Guessing it doesn't upgrade my JAL segment either?
5) If I'm traveling with a partner, and we want to sit together in Flagship Business, do we want seats A and D so that way we could get up during the flight and talk to each other? Or do we want D and H and there is someway to see each other or talk through the "wall" between the seats?

Thanks in advance!