I got an "end-plug" brake lever with that will fit bars with internal diameter 18.5–21mm. Does this mean I need a road bar (ie 26mm/23.8mm clamp/grip diameter) or can I use a MTB bar (ie 25.4mm/22.2mm clamp/grip diameter)?

It won't fit inside the MTB bar I have now (internal diameter 18mm), but I don't know if this is typical for MTB bars or if mine just has a thick wall. I don't want to sand it down due to the integrity of the carbon and I'd prefer to stick with the 25.4mm clamp diameter for greater flexibility in stem choice.