It was, in fact, TG's responsibility as the late-delivering carrier to reroute you to IAD. Your sole recourse is against TG and short of suing in a Thai or French court, you are unlikely to find any relief.
I would send a note to TG noting that you were delayed on arrival and misconnected as a result of a delay caused by TG, that you asked TG to rebook you (provide the name, description and location of the individual with whom you spoke), and that he refused to rebook. Accordingly, ask that TG reimburse the cost of the replacement ticket you purchased from UA. Keep it to that. It will almost certainly be denied. With that denial, make a claim against your travel insurance.
While I appreciate your views of AV/LM, but they might well have helped out here. As the ticketing carrier, they did not have responsibility for the reroute, but you can't know without asking. It forecloses one route for you.
If you want to try suing TG in a French court, go ahead, but it seems not worth it. unless you have the skills and time.
Last edited by Often1; Mar 8, 2019 at 10:38 am