The design of instrument approach procedures is based on all engines burning and turning. It is the operator's responsibility to ensure that engine-out procedures are provided for the pilots.
In this case, if you can achieve 2.5% after an engine failure, that is what you use. If you happen to fly a 146 around, then use the other figure.
The climb gradients do not start from the Missed Approach Point, they start from the "Start of Climb" point. There is a level segment in between with very reduced obstacle clearance.
Remember what is best practice. You should be following a vertical profile and not diving and driving. Therefore, you need to add your sink allowance into the MDA anyway. Remember to have your own visual descent point if it is not provided for you. Go-around after it unless you want to overrun.