Might be an issue with your editor. What WYSIWYG editor are you using? Try switching to default TinyMCE and see if issue persists.

1: On signup page, under email, I have this text:
"Please enter a valid e-mail address. A confirmation email will be sent to this address upon sign up."

Where do I change it? I tried the language overrides, but it do not pop up.

You'll need to use the language overrides feature described below.

www.joomlapolis.com/blog/kyle/18701-language-overrides-have-arrived

Sometimes language strings are partial strings. So just using the language finder for just parts of strings. That string however is using the following language key.

_UE_REGWARN_MAIL

This is just the default description text for the email field though so you can also change it by editing the email field in CB > Field Management and changing the description.

2: The banner photo on signup page. Where is it located so I can change it out?

Profile edit, profile view, userlist, and registration layout parameters are all a part of CBs template. So within CB > Plugin Management edit the Default CB template then under the Parameters tab you'll find parameters to adjust the layouts. For Registration you'll see a parameter to turn that image off or select a new one.