What is the nature of the dispute with your employer regarding the cold water sandwich issue?
The only way to truly eliminate the cold water sandwich is a storage tank for the hot water. There is always a flue purge delay prior to ignition once the flow has been detected, which both cools off any residual heat in the heat exchanger, and allows some water to flow through unheated before the burner fires. If that unheated water can be mixed with locally stored pre-heated water, the sandwich effect is pretty small. But there is no way to do it with an expansion tank, or exotic plumbing exterior to the unit that does not include some quantity of already-hot water. The amount of hot water storage needed is small, on the order of a quart or so, but it needs some thermal mass of pre-heated water. The larger the volume of stored water, the longer the ignition delay can be without experiencing a cold water sandwich. You can't cheat the basic physics.
In Navien's case it's a tiny storage tank and recirculation pump inside the cabinet. I wasn't aware that Rinnai had ever offered a solution. Due to increased standby losses, any storage tank solution cuts into the as-used EF efficiency, but it does fix the problem. To improve efficiency Navien made heating the internal storage tank programmable to time-of-day, on earlier models and it could be disabled completely if desired. But from a user point of view having to program yet another feature is a PITA. I haven't really looked at how the newest versions can be tweaked (if at all.)