1. in 'emergency load' mode, why does AP2 remain available?
Because Bell designed it that way. AP2 is on the Captain's side, which is also the pilot's side if flown single pilot, which the 412 is certified to do, both VFR and IFR. Bell's design philosophy is to maintain services to the pilot as much as possible/appropriate while regarding the copilot as "bonus" rather than essential.
ANS 1: no difference..ap2 remains on cz emergency bus 1 powers up inv 1 and inv 1 powers up AP2.
Not quite. Emergency DC bus 2 (which remains powered in emergency load) powers inverter 2 under normal circumstances, but the emergency load switch is connected to a relay which cuts power to inverter 2 in emergency load mode.
2. if eng no2 overspeeds, say its n2 103 %. no1's n2 is 100%. now the Nr (rotor RPM) would match no1's n2 as per the maintenance manual. how would the clutch (the free wheeling device) allow the low RPM (100%) to be matched with Nr against a higher RPM (103%).
If engine #2 N2 is at 103% and NR is at 100% then the #2 sprag clutch has failed to engage and the engine #2 power section is not engaged with the drive train (C-box output to the main drive shaft). The engine torque will read zero. A dangerous situation because of the possibility of a sudden re-engagement which will put a "sudden stop" type shock load through the engine power section, C-box, and the whole drive train. The porper procedure is to roll engine #2 to idle, establish single-engine flight parameters, and shut down engine #2. Do not try and restart engine #2 no matter how badly engineering want to see what is happening (believe me, some will try and insist).