I have a few hundred hours flying the BN2a26 (piston), I think VP-MON was a piston also? Regarding VMCA it is 43 knots. Climb on single engine with flaps up on one engine, propellor feathered, YVSE is 65 knots.
Clearly from the pictures the right engine was not feathered. Whilst in flight I have had a right engine loose power and yaw is significant however rudder, right to left everything forward (then left to right having identified, verified feather the engine). On my flight as I was in the cruise I trouble shooted before feather, I applied carb heat and within a few seconds (felt like forever)power was restored. It focussed my mind though. I have also shut down an engine (again starboard) at FL12 feathered the engine and she flies perfectly well albeit in this case I was in the descent - the piston Islander will only maintain 5300ft on one engine.
Water ingress. Our Islander suffers from water ingress espiecally after heavy rain. The seal around the fuel cap is the normal culprit (and yes we get it changed regurlary). There are 4 fuel drains on the ac. Often after rain we can remove several drains full of water - you have to be very careful you actually have fuel. Also due to the baffling water will rest inside the tank, sometimes we taxi the aircraft and do more fuels drains.
In this case it was reported there had been heavy rain just before the accident, a possible cause might thus be water ingress. If starboard engine failed or lost power shortly after take off the pilot must react quickly with considerable rudder and everything forward, flaps up, then the rest of the immediate actions. She should be manageable espiecally with a VMCA of 43 knots unless there were other problems.