First, the shear volume of US Operators means the FAA couldn't possibly review and approve an MEL for each plane. Heck, they're months behind on simple LOAs like RNP and CPDLC. I've heard of FSDOs taking six months to approve new requests. There is reason the FAA is trying for a workaround, they don't want it.
A very good point. Although if you can show it is pending an MEL being approved this should be no problem, since EASA issues temporary authorisations to use an MMEL for new aircraft.
almost no private operator has the technical knowledge ... to publish a MEL for each type operated
Is this not the nub of the issue? If they haven't got the technical knowledge to produce an MEL from the MMEL, how can they use the MMEL on a day to day basis?
the number of open MELs in the corporate FAR 91 fleet is vanishingly small
Unfortunately, 20 years of Part 91 experience tells me that this is more down to nothing being put "in the book" until the aircraft goes for maintenance rather than there being no snags on the aircraft. Again, the lack of technical knowledge comes to the fore, since often they are things that could not have been deferred anyway but people carry on flying oblivious to how they have reduced their safey margins. All in all not a good argument for the current system.