so, what solution you are hoping for here? This forum is best efforts and I think everyone on this thread, both paid and unpaid to be here, would love to help you resolve this issue and get filed, but it's unclear how if the issue is still the data we cannot see.
What sort of magnitude are you talking about in terms of number of sales, proceeds/cost/gain and difference vs. the brokerage 1099B?
Are you able to share more details, example screen shots etc?
Sounds like the Excel is bad. Do you not have your own original transaction information from the buys and sells, trade confirmations etc to determine the original cost basis and proceeds and associated gain/loss. Why has this become complicated - there can be issues with long-dated positions or account transfers where this information has been lost over time, corporate actions etc.
as for "Further, the brokerage can dismiss me as a client whenever they wish, with no reason, & then I'd really be screwed. " - they don't seem to be helping so if I had this experience I would run a mile.
