If with a duplex outlet you break off the "connector bracket" (Leviton Corp term--I always called it the duplex tab) next to the screw terminals it will separate it into 2 single outlets and using 2 sets of 14ga pigtails that should comply with code as stated. That way neither 14ga tap should exceed load specs. With the exception of the creativity of the human mind to intentionally overload it.
Nope. Generally you cannot downsize wire on the end of a circuit to below the breaker size. The only exceptions are in the tap rules, and they don't allow that on household receptacles. So wired as you propose, the breaker must be 15A.
For a 20A circuit, it would be a waste of material anyway, since an exception permits 15A receptacles on 20A circuits (specifically, only this case, not any others), and your two 14AWG pigtails need only be one 12AWG pigtail. (UL requires every 15A receptacle to be rated 20A for passthru). You can chain any number of 15A receptacles on a 20A circuit and be golden.
That scheme leans heavy on the concept that "the appliance itself will limit current on each endpoint". That makes safety conditional on everything working to spec. If so, you don't need any safety devices at all! So you can also toss grounding and all the breakers in the unsightly service panel. Splice right off the service drop and cover the splices in drywall. Done!
Obviously experience shows that doesn't work, so we build for failure.
If the circuit is a 20 amp circuit it would be a code violation to use 14 awg wire. You can use a single duplex 15 amp rated outlet on a 20 amp circuit because there are 2 outlets. A single 15 amp outlet on a 20 amp circuit would be a code violation. Your reasoning sounds good but if there was a failure the. 14 AWG wire would be the failure point..