With a Flex ticket, it looks like you get a refund when you rebook on a cheaper flight as long as the booking class is at least the same, or higher. This means if you book Q class HEL MAD and change the ticket to HEL ARN, you should get the difference in fare back, however you can only get HEL ARN in Q class or higher, and not on O or Z class which are lower booking class, that's how I understand it. I got a refund in such circumstances a few weeks ago. if you change the ticket to a higher fare, you can downgrade the booking class. And if you pay a fare difference, it is refunded when you refund the ticket.

Is there a way to book specific fare bucket (higher than cheapest currently available)? Want to book Y for a domestic flight to maximize adios earnings.

Have tried BookWithMatrix browser extension - looks like Matrix cannot find domestic AY flights, customer service also wasn't able to help.

Maybe in some another tool or online agency it is possible to choose higher bucket manually?

I have not been able to find a way to do this online. But AY customer service or most travel agents can sell you a ticket in any fare class. I have found AY chat the easiest, as long as you can get through to an agent.