OK I think we've firmly established that their brains are not functioning properly. Again, think of them as puppies. You are currently saving them from a long jail sentence. Absolutely the arson investigators would figure out immediately that they set the fire. Arson investigators are trained in all the ways that people set fires and have tens of thousands of photos in online libraries to compare against. There's no way your parents could possibly fool them.
Don't even try to argue with them. You've done the best thing possible - made sure they can't do it through publicity.
I know you like your plan of sending them on vacation but it relies on their cooperation to make it work. They've made it abundantly clear they will not cooperate. I don't think basing any plan on their cooperation is a good idea. I would make plans solely based on what YOU can do. You can't fix them or help them. They are on their own path. You need to focus on helping yourself.
You need a break. Your sanity requires it. Is there anywhere you can go even for one day to spend one precious day in peace?