Bicycle and motorcycle helmets won't really help. They're made to withstand moderate-to-high horizontal impact forces from a low height. A horse-riding helmet would be better. They're made to withstand higher vertical drop distances with less horizontal force, more similar to a fall from a ladder.

My mild acrophobia feels more and more justified...

Bicycle helmets have a minimum force value they help with to, so wont affect concussive worthy events only the bone breaking ones and being a skin barrier. (As person thats concussed in bicycling hill accidents). Dual layer helms are needed but not typically funded.