Have the compression tested. If it a bad valve you will find it that way. Any valve removal or replacement will require removal of the head, so get it to a tech you trust to get proper diagnosis. Trapped fuel would be in the manifold between the intake valve and the air flow sensor. When it gets hot, it vaporizes and can create an explosive situation (so to speak). This can cause damage if not corrected. Could be from a leaky injector(s). The hard start after the backfire is from an intake mainfold full of oxygen depleted atmosphere. Take it to someone you trust and get a good diagnosis.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.