No. Mi A2 does not have the removable battery. However, if required, you can replace the battery via authorized Mi service centers.
Yes, Mi A2 does support wireless headphones.
Yes, Mi A2 does have the fast charging support. It comes with 10 V / 2A charger in the box.
No. Mi A2 has USB Type C port and it also comes with USB Type C to 3.5 mm headphone adapter in the box. So, there will not be any problem to use Mi A2 with your 3.5 mm headphones.
The Mi A2 continues Xiaomi's now-familiar tradition of offering excellent value. The upgrades from last year make it an enticing option in this category, and the camera, in particular, is stunning.
The Xiaomi Mi A1 was an incredible value proposition in its market segment, the Mi A2 falls somewhat short. It takes away key features like storage expansion and universal headphone connectivity, plus it misses the mark in crucial areas like display and battery life. Affordable as it may be, it wouldn't be our default choice in the midrange quite like the A1 was.