agree.
Once the 'mounting turn is over, they do.Why is it simpler than just having them work as a unit?
If someone is pre mounted, I have them work on the same initiative order number because it's simpler that way for me as a DM. None of my players has yet taken mounted combatant. I gave it to a few bands of gnolls, mounted in dire wolves - they had the mounted combatant feature. Certainly made taking the dire wolves down a bit more difficult.
Yeah, any fighter, barb, monk, paladin of 5th level or greater gets hosed by the way extra attack is all of a sudden ignored whan an OA happens ...I find the added artificiality of readying actions (which is yet another nerf to melee combatants, the bigger the nerf the more attacks they have) far less simple.
yesI think you've solved the riddle; the turns DO overlap but it's important to determine which one starts first because of these effects that trigger on a turn's start. It's also why you can't switch it afterwards (or it would be abusable)