This turns off my users able to register which isn't what I am looking for.

You need to disable registration through Joomla. Once done you need to configure within CB > Configuration > Registration for CBs registration to be independent of Joomlas. This closes a backdoor and helps prevent some spam registrations.

I need to display the CB Login page instead of the Joomla login page.

You need a Joomla system plugin to redirect for you. You can find these at Joomla.org extension directory. Actually you should be able to use Joomla 1.6/1.7 built in redirect component found in Components > Redirect. Example as follows (this is just an example, adjust as necessary).

Source (should be SEFed):
index.php/login

Destination (should NOT be SEFed):
index.php?option=com_comprofiler&task=login

It's also possible to use an htaccess 301 (permanently moved) redirect such as the following.

Redirect 301 index.php?option=com_users&task=login index.php?option=com_comprofiler&task=login

Sorry, not much more I can advise. My previous post is the only tricks currently available to do this. Am not aware of a new "disable core registration" type plugin as found for J1.0 and J1.5.

The call to the Joomla login does. Is this a feature that CB doesn't support?

CB can return a user to 1 URL back using the &return parameter as seen in the URL, but Joomlas &return parameter is not encoded the same way ours is so it's useless to CB and no need to include it. This feature for CB basically only works when using the login module so it can remember what page the user was on.

I'm not a programmer but it seems there's a onAfterLoginForm triggered just after the login. Is it could be possible to create a simple CB plugin that catches this event, takes the return value from the GET and force redirect to these base 64 encoded page.

Of course, you could develop a new CB plugin to translate the &return and send a user to it. CB normally does this, but it only knows how to translate its own &returns which are not constructed the same as Joomlas. The below tutorial would assist in developing a CB plugin that utilizes CB triggers as well as provide a working example.

www.allmysocials.com/directory/tutorials/item/228-event-triggers