Don't think MAN for FR from Belfast is illogical at all. They'll want into the largest markets (London, North West, midlands) where lower fares can stimulate more traffic, which already exists but is driving to DUB for savings.
As for trying LBA instead of MAN, more seats to MAN on Airbus / Boeing at keener fares will disperse a good part of a large Leeds market away from flybe's Leeds flights over to Manchester. Yes, there are passengers who absolutely only fly to the east of the Pennines for sound reasons, but there are others who could as easily use Manchester v Leeds, and more, affordable choice to Manchester into an airport which has a more predictable weather history than LBA, not flying on a flybe prop will certainly appeal to those flexible passengers.
As for the argument about seat restrictions at BHD, they never took any heed of that constraint in the past when Ryanair were operating there, so don't see it qualifying as a reason why they are not growing now.