Dale Parker retired suddenly from all forms at the end of last year, not sure of the reasons but you may be able to find something if you go digging.
O'Shea will be in the team regardless, the four will be picked from the five in the days before, so the final spot in question is Edmonson's. Whoever Australia picks is still going to be fourth best behind the big three.
GB have Thomas back because he's the best they have, and they wouldn't be a chance without him (or Wiggins). It's hardly a trivial decision in any case, to put aside one's road career for effectively a season. For Australia it would be a lot harder because they would be competing for only one position, it wouldn't be great exclusively training the TP for six months only to not make the cut - I think even Meyer knows that.
The current group has broken the national record twice in two meets, they just rode a 3:53. Edmonson is young, O'Shea has only had one season back, they will all improve.
Credit to the team which wins gold by a few tenths on the day, two full-strength lineups, no what ifs.