Looks like AEC is intercepting the registration. Can't really advise you any further as I've no idea what triggers or behavior AEC is using. You can try setting the URL to index.php?option=com_comprofiler&task=registers, but no idea if that'll work either with AEC.

The form isn't repopulated though - can you point me in the right direction re keeping the form populated during the process.

Only other suggestion I can make is trying to error out the registration instead of trying to redirect it. First unpublish the redirect action then use just the below code action.

Type: Code
Trigger: onStartSaveUserRegistration
User: Automatic
Access: Everybody
Condition: [post_email] Not Equal To [post_cb_confirmedemail]
Method: PHP (create_function)
Code:Return: Silent

You can change the above error message to whatever you want. Hopefully this will do the trick otherwise you won't be able to keep the users already populated data. Directly after that trigger is an error check that picks up errors set like the above so it should redisplay registration with that error message at the top.

jQuery is client side, it's not secure; you really need to have PHP side usage, but at any rate you can use the below to add new jQuery to the page. It uses CBs already existing validation library to change the 2 fields to be comparison based.

Type: Code
Trigger: onBeforeRegisterFormDisplay
User: Automatic
Access: Everybody
Method: jQuery
Code:
Either of the solutions seam to be working fine for me, but again I can't say how it'd behave with AEC present.