1. If award space (ie. Award business I class) is available, you can upgrade.
2. Economy Extra is much better than UA E+. Bigger seats, footrests, better service, more amenities at the airport...
3. nothing special for you beyond normal *G benefits
4. yes you can choose a seat
5. your miles will clear just fine if it goes according to procedure (ie. you don't have to do a missing mile claim)
6. the A340 is fine- like a normal LH a340. sas economy service has been gutted a bit over the years, but Eco Extra and Biz have pretty good service...