MacOS App Resolution Independence I know how to change the screen resolution for the entire system.  That is not a problem.  What I would like to do (for my boss) is to "make everything bigger" in macOS 10.14 Mojave Mail, which is his preferred mail client. I know how to increase the size of the fonts in the mail list and in the massages already.  That helps some, but there is also the issue of the left-hand navigation pane (Inbox, Sent, Junk, etc).

Is it possible to increase the resolution of specific apps independently of the rest of the MacOS system?  Alternatively, how does one increase the size of other things in Mac Mail?
No, Apple does not support app-dependent window scaling. The screen is scaled as a whole. That's why it works so well, compared to Windows' attempts at HiDPI.

Sadly, Apple does not provide options for setting typefaces and sizes for a considerable part of its UI.

Your best bet is to set the screen scaling to a value where everything is at a suitable size.