After looking at some example projects including Lock Directory, reading up some topics like Detect Previous Version, and reading a few related threads including this one, I still haven't been able to figure out how to get the installer to do one of two things.

1) Refer to the registry value "InstallPath" (%_SB_INSTALLDIR%) that I have the installer create in "HKEY_LOCAL_MACHINE\SOFTWARE\*My product*", which would then replace (or skip) the "Select Install Folder" dialog since an install directory has already been determined during a previous installation. This is my preferred choice.
2) Prevent users from picking a custom installation path themselves, thereby being forced to install to the path I've specified.

I may have not figured this out yet, despite the resources, because I'm quite new to this stuff and the answers didn't make enough sense to me to pull it off. I'm not a developer myself (quality assurance), I was merely handed the installer task by my company, so you may have to be gentle and be very specific. Hopefully someone can help me.