Q: 403 when going to localhost/~<username>

HTTP 403 is a protection error.   Look in the Apache log files and see what access it's complaining about.   The access and error logs are probably in the /var/log/apache2 directory, but I run OS X Server here and not OS X client with Apache manually configured and the log locations are different between Server and not-Server configurations.   As a guess, the 403 error is probably arising on the index file (usually index.html) for the directory path you've specified for the keeffefamily user.   That directory and that index file needs to be at least read accessible to the www (_www) user, or whatever user you're running Apache under.

There are a bunch of results when using this search string in Google: Apache localhost 403 forbidden

Here is one of the many.

Hello again DaCake32,

It sounds like you need to enable mod_userdir. See here: Setting up a local web server on OS X