With regard to whether a company can operate Part 91, this is what the Kidd letter said:
Your next two questions involve a scenario in which a certificate holder operating under part 135 "manages" an aircraft for an owner and conducts flights on the aircraft for the owner's personal use or that of his employees. You ask whether the certificate holder can operate these flights under part 91 rules and, if so, whether its flightcrew members must still comply with the rest requirements of§ 135.267. Because these questions are matters of compliance assistance instead of legal interpretation, we are forwarding them to the Air Transportation Division, AFS-200, of the Flight Standards Service for a response.
I other words, the Kidd letter does not answer the question as to whether a pilot can operate Part 91 after dropping off the medical crew and/or paitient (equivilent of payig passengers). The question is not answered, making the reference to Kidd about this issue moot.
Stop saying "reference the Kidd letter" since it is not relevant your assersion that a HAA pilot cannot operate Part 91 after dropping the medical crew and patient, thereby not restricted by 135.267 duty limitations!