I will make a mid-year Roth conversion. Can I make a lump sum estimated tax payment at the same time to cover it, or must I divide it over the remaining quarters?

By using the default of having the income treated as received equally throughout the year, you might end up having an underpayment for an earlier tax quarter, depending on your tax payments relative to your previous year's tax liability.  If using the default treatment would result in underpayment for an earlier quarter, you would need to annualize income on Schedule AI of Form 2210 to avoid that penalty.  Annualizing income would result in the income from the Roth conversion being recognized in a single tax quarter, so spreading the estimated tax payments over more than just that quarter could result in underpayment for that quarter.  If you are unsure whether or not you would need to annualize income to avoid a penalty in earlier quarters, it would probably be best to make the estimated tax payment all for the quarter in which the Roth conversion occurs.

The only other way to avoid the possibility of underpayment for earlier quarters under the default treatment would be to have the taxes withheld from the Roth conversion, then complete an indirect Roth conversion of the portion withheld for taxes by substituting other funds.  Unlike estimated tax payments with are always treated as paid when actually paid, tax withholding is by default treated as paid evenly throughout the year.