For the record IAG applied to the EU regulators to 'merge' BA and BMI LHRPony so it is fairly certainly a merger.
BA is making decent profit and BMI is making (for it's size) phenomenal losses. Part of the proposal made to the IAG board by the BA board was that the purchase of BMI would be 100% funded by BA (and hence profits BA pilots helped make). This is a successful airline 'taking over' an unsuccessful airline that is for all intents and purposes, bankrupt. How this should affect the relative negotiating positions of the two pilot groups is perhaps up for debate but any notion that this is a 'merger' is ridiculous.
The BAcc wishful thinking that they can 'stitch-up' the pilots coming from BMI pre-merger will fall foul of their expectations once the deal is approved.
Again, the BACC has no hidden agenda and is not trying to stitch anyone up. I would strongly suggest you will fare out better by heeding their advice. However, that is a choice you will have to make. If you/BMI-CC choose not to listen to the BACC and things do not work out as you had wished, I hope you remember the advice that was offered and ignored.