There is a forms portal component that will allow you to set search parameters to display Adaptive Forms and XDP forms.  AEM 6.3 Forms Help | Creating a forms portal page

Patches for AEM are found on packageshare, JEE patches are made available through support.  Enter a support request for JEE patches

Put a script on the button to validate that the required fields are filled.  You may need two buttons, one that the user sees and clicks on and once the validations are done it executes the click of the submit buttons.

I don't understand what you're trying to do.  Is this a form created in Designer or Acrobat?  If you're trying to combine the two PDF files this can be done in Acrobat.

AEM 6.3 Forms Help | Embed adaptive form in external web page

You're attempting to created an interactive XFA based PDF form.  Are you going to be trying to load this in a browser?  If so you'll run into issues, you may want to look at rendering the form as HTML5 or using Adaptive Forms.  Let me know which direction you want to go.

Looks like you're not using one of the supported JDK's as the message suggests.  Looks like you're using openjdk, please refer to the following for supported JDKs Technical Requirements

I assume you're running the standalone version vs. JEE.  Also are you creating a non-interactive PDF or a form to be filled.  If it's a form I'd not recommend doing this and switch to an HTML rendering of the XDP or look at Adaptive Forms.  The javadocs are available here AEM Forms API

Recreated the issue looking into it and will report back.

Yes there are a variety of ways to do this, you want to create a document of record which is defined when you create the form in the form model properties you can automatically create a document of record.  You can also add a form submission object on your form and it will give you a link to the doc...