Second - yes, sure, just like with (almost) any other paid app.
Google saves your purchase history, so just
1) configure the new device with the same @gmail account in system settings, so that Google Play knows it's you
2) install the base app
3) install the unlocker, which will be marked in Play as "purchased" (after pt. 1), so you won't have to pay again
Please see this on the main site for further info:
http://www.aqua-mail.com/?page_id=206There is nothing in the Play version of AquaMail's unlocker to limit you to one device (e.g. IMEI, serial number checks, etc.)