You could also do most of this trip as a DONE4 originating in Europe ... which would cut the fare by a lot (circa 30% from the right city, I haven't bothered to check exact pricing). The routing would be in a different sequence but the only stopover you give up is SJO. Europe-South America-Australia-Asia-Europe (either eastbound or westbound).

As for your questions,
1) valid
2) unlimited surface sectors on OWE (unlike most other RTW fares), however it makes ticketing difficult for most systems and might require the ticket to be handwritten.
3) the choice of airline varies by route, date and equipment type. LA is introducing a new J which is far better than their old J, for example -- so you should pick AA over LA old J but LA new J over AA, for example. There have been big improvements in LA/IB/AY in the past few months, making them more competitive with QF/BA/CX/AA.