To be real picky, the US DOT rule does allow the rules of the first marketing carrier, *ignoring any elite or credit card benefits* for the subsequent airlines.

So TG could be within their rights, even as a USA-flying carrier (even though not on your itin) that has to play nice with the USA, to give you only what UA would give a non-elite.

I believe that the IATA now has a database and the etickets now have a field where the carrier who determines the baggage rules and charges stores them, and all carriers in the "journey" (the term in the rules - not "ticket", not PNR, but I guess instantiated as "ticket") can access them.

To make it even more confusing, the USA DOT rule does allow the US-touching journeys to be charged based on the MSC Most Significant Carrier concept of IATA res 304, as long as the MSC is modified by the US regulation saying "use the marketing not the operating carrier if they differ", and "apply the outbound rules on every leg of the journey" - but made that optional. United and most USA airlines have chosen to ignore that option, because they can make more money using their own more expensive "originating carrier" fees.

The idea of the IATA MSC rule was great. The idea of the US DOT rule was great. The lack of coordination between them and the MSC-as-modified-is-optional of the DOT, has made this a total crapshoot every flight if any of the airlines have anything to do with the USA. Unintended consequences strike again.