I dont know which one of your questions you’ve solved but here it goes:
-Detach the spawned particle system before you deactivate the flamethrower so that it keeps burning even after you leave the trigger.
-You can use a Gate > plug OnBeginOverlap event of the Trigger volume to Enter > plug BeginOverlap event of flame damage volume to Open and EndOverlap to Close. This way you will receive damage only when you are overlapping both volumes at the same time.