Hello again rg305 and griffin. I am happy to report some more good news. With the kind help of the developers of the web server software that I am now using, the HSTS and redirection issues have now been resolved. What had become a really frustrating and aggravating nightmare with MAMP PRO, is now water under the bridge. As of this date, both of my sites now have:
Automatic renewal and installation of LetsEncrypt certificates.
Proper automatic redirection from http to https
Both sites qualifying for the HSTS Preload List
To any macOS users who are reading this, if you are having problems with MAMP PRO due to the many changes they've made in recent updates which forced us to do certain things THEIR way, I strongly recommend that you switch to Abyss Web Server. It costs the same as MAMP PRO, but their level of support is so much better -- they actually give you timely responses -- and they will help you to work out your issues until you have a smooth-operating site. Plus the automatic installation and renewal of LetsEncrypt certificates relieves you of the hassle of having to do it manually every 60-90 days.
Thank you again rg305 and griffin for trying to help me. I appreciate it.