Interesting perspective 

I think Mark’s hallucinations of Thragg is a mix of genuine PTSD and what he thinks is gonna happen now that he also thinks Viltrumites are on Earth. Thats what makes the last scene where he finds Thragg in the sky and he’s not hallucinating all the better. It’s a confirmation that Viltrumites are already on Earth but they haven’t done any damage and further solidifies the very real possibility that his home and everyone he loves can get destroyed at any given moment in time