Is an EZPass Maryland transponder valid for a vehicle that was originally registered in Maryland but which is now registered in Massachusetts? ... assuming that the MA plate number would be provided to EZ Pass MD

That is correct in Massachusetts. Different states have different rules regarding out of state tolling . MD works in Ma, not sure about the other way

Was advised by EZ Pass MD that the EZ Pass MD transponder will be able to be used with a Massachusetts-plated vehicle but that the new MA license plate # will need to be added to (will need to replace the MD license plate #) the account. Can anyone please confirm this?

Should work especially on I95