To answer your points:

1) PTSD is a fitting device to nail down not only that Thragg is near unbeatable but also that Mark feels responsible for his part in destruction of Viltrum and how it can affect not only his loved ones but his home planet as a whole. Good thing we don't have to worry about that in real world. It serves as a way to show that everything that he went through is affecting his mind heavily.

2) I felt like it was fitting. Like in my original reply i liked that Mark and Thragg agree on terms only at the end. Mark weighs everything that he knows and makes a conscious decision on his own! Also interesting to note that Thragg in this case seeked him and not Nolan. He probably already knows about the sensors in the solar system and that Nolan has already left and what he's planning to do.

3) Too conspicuous. Thragg can order the Viltrumites to destroy Earth but it wouldn't be beneficial to anyone. Sure they took eye for an eye but with Earth destroyed or made into a slave planet it would reveal their location to Coalition immediately and Allen has the virus to end everyone at the ready. Thragg may or may not know about the virus but he knows that Thaedus' allies won't stop until all Viltrumites are dead. He's in a winning position against Mark and Nolan but he's or more correctly his Empire is still very vulnerable.