First I would suggest that you do not speculate in a public forum about whether some of the damage could have been caused by repeated seepage of water. (This is typically excluded.) You do not know this is the case, and the mold could have easily resulted from the recent water damage.
In Florida, sewer backups are typically excluded from coverage. However, there could be coverage for a sudden and accidental discharge of water from a plumbing system if the cause of the backup was on the insured property. I do not know how it is in California, but you may be covered even if there is an exclusion for sewer backups.
Unfortunately, California does not have an attorneys' fee statute for insurance law suits. In some states, Florida included, the insured is entitled to reasonable attorneys fees and costs if they prevail against an insurance company under a policy. In California, it appears that you can only get attorneys' fees if the insurance company acts in bad faith.
My suggestion is to speak to an insurance attorney in California. You may be able to file a declaratory action and have the courts determine whether there is coverage. Unfortunately, you may have to end up paying your attorney a percentage of the amount collected unless you can go after your insurance company for bad faith. It may be worth it since the amount of the loss seems very large (and you may be able to discover even more damage with a thorough inspection).
