Identifying SOT-23 IC: N59M
If you can't find a better match I would suggest giving it a try with an AO3401A 3A 30V p-channel MOSFET. That matches the voltage rating of the APM9435 MOSFET used in the other SMPS channel (sources are tied together according to the PCB layout), at lower current (~3-4A) and is also logic-level gate. 

Of course it could make things worse than they are now, but I doubt it.