1. The only thing that the custodian would be able to confirm is the calculation of the RMD from year-end balance for the preceding year and the appropriate divisor from the Single Life Expectancy Table in IRS Pub 590-B. They may or may not be cooperative in doing so for the older RMDs.
2. There is no real benefit from taking multiple distributions. No matter how many you take in a single year from a particular account, they'll all be aggregated on a single Form 1099-R. Just retain your records of the calculations used to come up with the total.
3. Income taxes on these amounts have not yet been paid. The distribution in 2020 must be included on your 2020 tax return and will factor into determining your overall tax liability for 2020. Any amount that you choose to have withheld from the distribution for taxes (default 10%, 0% or more than 10%) will be credited on your 2020 tax return, reducing any balance due and any excess withholding or estimated tax paid will be refunded. Just be sure that you either have enough withheld or pay enough in an estimated tax payment to avoid a tax underpayment penalty.
