FWIW, ACA only allows 1st of the month coverage. You probably already found this out. Short term plans are available for any effective date you want but they are subject to underwriting and pre-ex limitations.
For anyone else that has loss of coverage as a qualifying event that happens mid-month, I would (personally-if it were my family) would enroll early and let the carrier catch the error (or not). Carriers habitually have not returned ACA premiums and changed the effective date retroactively when they have provided coverage. You are no worse off if they do catch the error unless they force a termination completely. I haven't heard of any cases like this but it's possible (perhaps). I have heard where worse clerical errors have been made by those working for the exchange.
For anyone else that has loss of coverage as a qualifying event that happens mid-month, I would (personally-if it were my family) would enroll early and let the carrier catch the error (or not). Carriers habitually have not returned ACA premiums and changed the effective date retroactively when they have provided coverage. You are no worse off if they do catch the error unless they force a termination completely. I haven't heard of any cases like this but it's possible (perhaps). I have heard where worse clerical errors have been made by those working for the exchange.