One other step for the OP to take: once or twice daily snapshots of the on-line account information (preferably at same time as on-phone measurements). It might show what window of time is involved, and could even show an impossibly high rate of data consumption. That will be useful in getting charges reversed.
If at all possible, port out to another provider as soon as charges are reversed. There is possibly a defect in the billing system and, from reading posts here for years, those don't go away quickly.
If you can't port out without huge charges (which frankly are no bigger than some of the overages you'll be facing), at least change handsets for a while so that you can contradict Fido's attempts to blame it.