I do not know how to answer a hypothetical question like that with a BD that a member of SEC/FINRA/SIPC with very strict reporting standards.
If you're really worried use multiple brokers. It helps protect you from other things like IT failures shutting down a broker.
Hypothetical yes, but in the case of FDIC, history showed that FDIC always made the depositors whole even for those that exceeded the limit. So, I have no problem doing business with a small local bank even when my deposit exceeds the limit. If we can be somewhat reassured that SIPC can act in a similar fashion, I would be more inclined to consider smaller brokerages because they often offer better service.
My info may be old (years ago I was a Principal)... but the way the regs read back then regarding this... "SIPC insurance covered stocks/cash MISSING" from an account... for ANY reason... up to the limits of SIPC coverage.