Magento – Move login pop up form above shipping address magento 2 checkout

Magento 2 documentation has now been updated to answer your question, they cover more areas which may help too:

http://devdocs.magento.com/guides/v2.0/howdoi/checkout/checkout_overview.html

However, they have a section specifically on adding a form (where you can gleam how to add a field using the same method).

http://devdocs.magento.com/guides/v2.0/howdoi/checkout/checkout_form.html

The most pressing issue that you first must understand is that the methodology for adding the component or input depends upon whether the form is static or generated dynamically. The address fields in checkout are generated dynamically whereas the above link (adding a form) shows how to add a static form (or fields) by adding your uiComponents with the xml layout.

Adding a field to a dynamically generated form will mean a plugin on the checkout layout processor* and modifying an array through at least ten keys for some reason. More details in link below:

http://devdocs.magento.com/guides/v2.0/howdoi/checkout/checkout_new_field.html

• Layout Processor: \Magento\Checkout\Block\Checkout\LayoutProcessor

You've asked a bit of a mouthful there -- probably too much for a single Stack Exchange question, so I'm going to concentrate on teaching you how to reset the initial state of the form so things no longer appear invalid if there's a blank value. If you're interested in the whys, you'll need to get up to speed on the fundamentals of UI Components and x-magento-init scripts. My Magento 2 UI Components and uiElement Internals series are a good place to start. The following assumes you have some familiarity with these systems -- if there's something confusing below then the above articles (as well as the other Magento 2 articles) will probably have the information you're looking for.

In the Checkout Application, each form field is represented by a view model, and one-or-many view model templates. You can see this in Commerce Bug's KO Scopes tab.

The firstname's view model is an object derived from the constructor function returned by the Magento_Ui/js/form/element/abstract RequireJS module. You can find that model's source code here

vendor/magento//module-ui/view/base/web/js/form/element/abstract.js

The reset method is the one we're interested in. This will reset a field's validation state. You'll need to figure out the full uiRegistry string identifier for the specific model (again, Commerce Bug is useful here), use that string to fetch the instantiated view model, and then call the reset function. For the checkmo form, the following program (easily adapted into a define module if you're going to use it in your application) will do that for the firstname field.

requirejs(['uiRegistry'], function(uiRegistry){
    uiRegistry.get('checkout.steps.billing-step.payment.payments-list.checkmo-form.form-fields.firstname').reset();
});

Of course, you'll likely want to fetch each view model that's part of a particular form -- the following is a start on that.

uiRegistry.get(function(viewModel){
    var parentName = viewModel['parentName'] ? viewModel['parentName'] : '';
    if(parentName.indexOf('checkout.steps.billing-step.payment.payments-list.checkmo-form.form-fields') === -1){
        return;
    };       
    console.log(viewModel);
});

This uses the uiRegistry's callback feature to search for all view models with a parent of checkout.steps.billing-step.payment.payments-list.checkmo-form.form-fields. Some of these may not be form elements, but we'll leave that problem as an exercise for the questioner.

Hope that helps!