Is it safe to split a NEMA 10-50 receptacle?
The grounding/GFCI scheme sounds plausible. However, you still have a big problem that the 15A receptacles will be on a 50A breaker. They must be on 15A or 20A breakers. Anything larger and you risk a problem of an appliance fault that doesn't cause a ground fault but results in 2X overcurrent (i.e., 30A or possibly more) never tripping a breaker and instead causing a fire from overheated wiring.

In addition, there are code issues with removing and/or piggybacking on the range circuit. But ignoring that (e.g., if you simply unplugged the range and heated everything in the microwave) you still have the true safety issue of 15A/20A receptacles on a 50A circuit breaker.