DESTINATION ALTERNATE
 

When flights are dispatched with a filed destination alternate and should there be a need to change this to another one in the air, is it mandatory to inform the ATC? Can the ATC refuse to accept the request?

After God only you is the master on board not ATC...go where a valid reason takes you!

ATC is part of ATS-Air Traffic Services. They may NEVER make an operational decision for you-that is ALWAYS the crews prerogative. If they say Clearance not available and you need it guess what -you declare PAN and go anyway!

been there, done that

I believe this would only apply in case of communication failure and destination being closed.
So then, you must try to follow what is filed. Try, as other variables may be more relevant then follow this rule. So, we do what is needed in our best judgement.

Who decides on an alternate at the briefing? You or the O'Learys of the airline?

For dispatch only the destination alternate has to be filed with ATC, once in the air you decide where to go.

When I have been stuck in the hold etc with a high probability of a divert ATC have always asked me my intended divert destination. I have never been told go to filed airport, that’s in Europe.

Except in countries that exercise operational control.