The problem is, the airport line didn't construct a loop track (or siding) in airport stations to allow trains turning back. Airport trains heading to the CBD will continue its service as T2 inner west line so it's impossible to use exclusive airport line carriages.

How about those 77W aircrafts virgin currently owns? 

It's an ex-Hainan Airlines 787-9 with exactly the same cabin product. 

Is there any chances to introduce E190-E2 in their future fleet? Although it may be cheaper to purchase used aircrafts at this time

Maybe they just stick to the original planned schedule, replacing some ageing A320s with neos. However their 20-yr-old A320 I took this year still looks quite new.