Hi Bill, good morning.
A few thoughts first, when I install Windows 10 for sure did it on it's own fresh partition.
Fresh installs are the best, but upgrading is okay too for convenience not necessarily accuracy, as well you get that huge GB windows old folder to carry around till you dump it (but actually as a test I might even do an upgrade myself when 10 is a little more seasoned) ..it's as of right now on my list because I want to track exactly what a fully loaded Win7 to Win10 upgrade process would do to all my WinMail 'email message store' files located at:
C:\Users\name\AppData\Local\Microsoft\Windows Mail
I've not tested a fully loaded 7to10 upgrade way yet, only fresh installs and 10to10 upgrades, but I suspect all the upgrade does just like I've noticed 10 to 10 upgrades do, is to over-write most all the files in the WinMail Programs folder, and you would just have to Take Ownership of it again, and copy back in place your saved 'working' one, and merge WinMailEdit.reg for gp ...all the same info is on post #1 though.
I would always suggest to create a 'partition backup' of your original Win7 install first before upgrading! ..that way if you want to roll back as you say, you do it more precisely by, restoring the partition image with no install leftovers, or any other standard roll back anomalies, and at the same time you still have a partition image of 10 to restore right back to where you were, still having 7 !
Not sure if you're 'in to' using imaging software or not or have that flexibility, I would hope so, and of course then before doing any major upgrade you create a backup of where you're coming from first so that no matter what happens, you are guaranteed a way back precisely byte for byte to what you had before; also you can restore various saved time-line partition images of whatever OS you want to play with. I don't mean to digress but the thought of you rolling a Win10 installation back to Win7 just because of WinMail seems a little bit drastic. Also it's good to have working OS's of both OS's and not just depend on one upgrade process and that if you'll even like the different feel of 10. I find myself booting back to Win7 all the time still.
If you don't have any partition imaging programs, that's fine, let's get back to WinMail and your questions:
~ On x64 versions, no don't use that (x86) folder, only use the "C:\Program Files\Windows Mail" folder
~ When I referenced WinMail on 8 is different than on 7, what I was specifically referring to is that it's only just ''the process to reinstate WinMail is different on each OS'' ...but my saved 'working' WinMail programs folder itself and all it's files within that I used, are always the same folder! ..whether I transfer from Win7, to Win8, or to Win10, they are identical and are the original first modified folder to get WinMail working on Win7 in the first place. The only exception of course is if your OS is a 32-bit vrs 64-bit OS, and so naturally I keep two sets of working WinMail Program Folders, one is for my 32bit OS's and the other for my 64 bit OS's because They Are Different, but in each case yes they go to the "C:\Program Files\Windows Mail" folder location - hope that clarifies it.