Squirrelmail is just fine, and you appear to need to manage users, passwords and such, with plugins. I use RoundCube for myself only. It's not as advanced as Squirrelmail yet (still alpha/beta, no user management or plugins), but it suits me just fine when I can't use straight IMAP.
Just continue with Squirrelmail, and make sure you configure it the way it should be done. It's not rocket science.