Yeah, the ditching and no gear checklists have you pull breakers in addition to the GPWS override.

Can anyone confirm that the no-manual landing gear extension is an FS limitation?

I think it depends on how pmdg simulated it as fsx by default have the manual extension by using shift+G.Other softwares are able to extend/retract gears from hand pump in the cockpit (a famous spitfire for example).So, if there is a limit it is because pmdg did not used fsx default gear simulation.I hope that pmdg will clear doubts about it