Question 1: I believe that you would fall back to silver (Somebody may be able to confirm). However going back up to Gold requires 24K miles, which is double what you would need right now. Would your trip to TPE cover both the 4 Aegean segments and the 24K miles required?

Question 2: Is that one-way or roundtrip? Is the stop at AGP and BKK required, as it will drive up costs. In any case AGP is not a natural transfer point for Star Alliance flights so that will likely mean that you would have to break your trip there - making it two tickets rather than 1 (Which will likely be substantially more expensive). Do any of the flights, on the dates that you want to travel, have any award availability?

Question 3: If the person that you are traveling with has no interest in collecting miles I would try to spend as many award points on them as possible. Award miles devalue and any chance to redeem them (while getting a reasonable return of 1+ euro cent per mile) is one that I personally would go for.