Hi John,
From your screenshot background that looks like x64 right, you might want to change your spec to say x64 instead of just Win 10 Pro. Sometime these details are important for sure.
Speaking of screenshots you did a good job of outlining your problem.
I've never encountered that particular problem myself, but there was someone recently that pm'd me with a similar message store prompt message, but I don't know if they ever got it resolved.
It's always something simple, however it might not be in this case because of a number of reasons and what you did to cause this issue. Thinking outloud here, if you imported you old message store into the new one and clicked some wrong buttons, or some other function similar you did setting up WinMail.
Also I myself triple boot on each of my Primary partitions, and yes of course WinMail is on all my different OS's since Vista, However! in my case, I don't use a standard or definitely not the built-in boot loader of the OS, no way do I want MS or any particular OS boot loader system files getting its paws into any one of my other OS installs; iow, installing OS's allowing it to place 'any' of its OS files throughout any other of my primary partitions or even logicals is strictly not allowed on any of my setups! period! ...specifically because for those very reasons. On each Primary partition of mine, only One OS is allowed to be set active and set visible at any given time, the other two are Always set Hidden. When I install a new OS - all other primary partitions are set in-active, and set hidden, so that the built in boot loader does not see or activate, AND, it does not place any of its files within any other primary partitions. When I boot to other partitions OS's, automatically the one that it's leaving, is set inactive and set hidden (...anyway, all of which is very OT to WinMail so I don't want to digress here)
Back to your issue, in order to solve your current problem I need to be able to reproduce your problem, and then develop a reverse for it. I would need to have your exact movements of what you did to duplicate it, and then resolve it, and/or your computer and keyboard in my hands to figure out how to set it straight.
Also as you know I am a big supporter of everyone use a partition image backup software program, so that they can restore OS partition images at will, backwards or forwards in time, which allows them to have god like power over most all situations - I wish we could do that in real-life with our lifes' timeline - LoL
You should only have One:
''C:\Users\you\AppData\Local\Microsoft\Windows Mail'' folder and its specific related files within it's folder.
(once you start fooling with files or accounts in this folder, or news or local folder (needs to be done only by an Expert user) ..then all bets are off, but once you do and the registry picks up various incorrect targets, then again you're in trouble! Some type of registry and/or system and/or partition image restore is necessary to set it aright again.
I do know one thing for absolute sure, is if you install your OS anew, with it being the only OS seen on your system, and following proper importing guidelines, fresh installs are better, then absolutely you will be able to reinstate WinMail every single time without any problems.