Welcome
1) Not sure what you mean by true J - you will travel in Bus Class, get miles and q-points for J. The only difference is the price and that your seat comes out of the D fare bucket.
2) Yes - others have commented on doing so - only on AA flight #s and AA metal (ie not AA codeshare on QF metal to Australia)
3) yes
4) No - combinations with other tickets is perfectly fine - it does not violate the xONEx ticket rules
5) these are numerous - incl mile-maximization, status maximization, trying all of the better airlines (BA/CX/QF), trying the best lounges (Pier/Wing), and many more. You will be able to read many aspects of these on this forum - if you can't find something ask specific questions here.
Have fun planning and travelling.