If the DIY work wasn't the cause for the fire it shouldn't matter, but I half-expect someone to inform me that US insurance companies can (legally) deny coverage for reasons unrelated to the accident.
Not so fast. Have you very carefully read the full small print of the insurance policy? Did you review that with a lawyer? Is incredible how different "normal" people vs. lawyers can understand a contract.
I'm pretty sure there is a clause, which states that you have to inform if you have and/or are not allowed to have fire loads, or anything that could cause a fire, or make it worse, or something along the lines in legalese. These formulations are always there because of people hoarding fuel in the basement, for example, or O2 Tank, or whatever. They are formulated in the most generic way possible to catch anything you do "wrong". Failing to follow such clauses, also when not explicitly stated, is dropping your obligations in the contract. And then there will be a clause that of course says, that not following the contract from your side, also exempts the company of paying.
Note also there are clauses that are very softly specified, like "use rooms for the intended purpose" which may be a problem if you store idk, paint in the garage, which may be flammable, in which case a fire in the garage will not be (at least fully) covered.
