BigIP HA Cluster upgrade to 12.1.2- loosing DeviceTrust

I would say yes you can simply force the Active device into offline state as the Standby should still be getting heartbeats and when they stop will go active.

The Device Trust not working should only effect the syncing and possible sharing of connection state data, so likely you will not have any mirrored persistence records or connections.

I do recommend testing this out if possible in a none production environment, even if you setup a couple of F5 VEs see if you can replicate the test before.