OK, got Squirrelmail's blank page sorted as well:
[user@example ~]$ sudo /usr/share/squirrelmail/config/conf.pl
Within, I simply went to 8 (plugins) then selected at random one of the ones I'd installed, in my case 5 (change_sqlpass) to disable, saved and quit, and hey presto, Squirrelmail is now loading! Yay! I'm not that troubled about this plugin for now so am gonna leave it disabled; however, I am still having some SQMail issues in that it doesn't seem to be downloading new messages. I can successfully send from it but the last e-mail I received was from Tue 22nd (yesterday); today is Wed 23rd and indeed Last Checked is confirmed as "Wed, 10:04 am". Still, this is really a different issue from what I originally wrote about so am gonna mark this as solved and carry out separate investigations for Squirrelmail not currently receiving mail.