This has nothing to do with M&M. You are simply credited with whatever the operating carrier credits. TG's policy is not unlike many other carrier's policies as to airport and other last minute UG's.
As practical advice for OP, consider that many employers and their corporate TA's are more than willing to accommodate employees by permitting the use of a second personal credit card. In this scenario, the cost of the allowable fare is charged to the employer and the balance is charged to the employee's personal credit card (with the employee's authorization). The ticketed and flown cabin are thus the premium cabin(s) as selected and are not upgrades of any kind.