disable continue until user has entered mailadress

It looks like you need custom jQuery to do that. I don't have time right now, but have a look at the following articles to help you write a custom solution:
http://stackoverflow.com/questions/5614399/disabling-submit-button-until...
http://stackoverflow.com/questions/4473173/how-can-i-disable-a-drupal-fo...

You'll need to think about all possible situations:
- the user is already logged in, so email entry is not required.
- the user's email address does not exist on the system.
- the user's email address does exist on the system and is not logged in.

Good luck

I haven't run into this problem but I can imagine it.

Sorry to say that I'm far from proficient in Drupal, let alone Drupal Commerce. I didn't quite get a grasp for how the ajax + forms were working so what I wound up doing for other pages is using Checkout Extra Panes module to insert a node with whatever JavaScript I wanted (not for this particular bug/problem, though). As much fun as that was it makes

Is there a way in Drupal to disable/enable submit buttons in regards to validation? E.g. if these fields have not passed validation via ajax they shouldn't be able to submit in the first place? This should be applied to both the Login Now button as well as the Continue button.

Check out the AJAX parameter for form elements:

http://api.drupal.org/api/drupal/includes%21ajax.inc/group/ajax/7

Hypothetically you just change the form in the place that the ajax is being assigned, i.e. button disable/enable, and in the custom ajax callback you return the form element/field/whatever that was already updated in the previous function. Then Drupal's AJAX framework updates the HTML/JavaScript just based on that.

In other words you don't even touch JavaScript! It's pretty impressive. Like I said before I don't quite get it myself just yet but I think that would be the route to go.

It's used in the code for this module already so you have a working example to play with.

This brings up another usability issue I just remembered... user punches in e-mail address...goes to e-mail address field... leaves... and it does a check on the e-mail refreshing the form even if they didn't change anything. That's a usability no-no. After you figure this one out we can patch that up, too, pretty easily probably.

confirmed.

1.)enter a already registered mail
2.)password field appears
3.)click somewhere outside of the field
4.)rechecking already checked mail

is there a way to check "if field value has changed" using ajax?

try to add the below javascript into a .js file and add it to the commerce_checkout_login.info file like scripts[] = .js

jQuery(document).ajaxStart(function() {
if(jQuery("#commerce-checkout-form-guest-user-checkout").length ){
jQuery("#edit-continue").attr('disabled','disabled');
}
});
jQuery(document).ajaxStop(function() {
if(jQuery("#commerce-checkout-form-guest-user-checkout").length ){
jQuery("#edit-continue").removeAttr("disabled");
}
});