If the frost line depth is 4.5 feet and the bed rock depth is only 2 feet, is it ok to build footings and a foundation wall on 2 feet of compacted 3/4 crushed stone?

The 2 feet depth of earth around the perimeter of the crushed stone is a 'sandy/organic mix' that is moist to wet - wet when there is a lot of rain or during the Spring thaw. There could be a 'french drain' between the crushed stone and the perimeter earth.

Will such a footing/foundation construction be safe from the effects of frost?

Sounds like "non frost susceptible fill" which is normally OK as long as it always drains. Or you can do a frost protected shallow foundation in which case regular fill works too. But of course check with a local engineer.