I did some more research and tried an experiment, that ends either by typing "sudo killall httpd" into Terminal, then restarting only my newer (MAMP) Apache installation or by simply going into System Preferences and turning off Web Sharing. In either
event, I can still view phpMyAdmin at http://localhost:8888/MAMP/frame.php?src="">
And if I open up my MAMP icon and kill my Apache and MySQL servers, then I can no longer view phpMyAdmin. So my second Apache version must be working, right? If that's the case, then all I have to do is figure out why my virtual hosts aren't working with the newer Apache version, and I should be back in business.