This is to completely correct.  Data from a form is stored in a database.  Documents such as attachments are typically stored on the file system.

https://helpx.adobe.com/aem-forms/6-2/prepopulate-adaptive-form-fields.html

Yes you can render an XDP form as HTML5, note that this rendering is NOT responsive it looks or mimics the PDF rendering of the form.  You CAN however use the XFA form as the basis for an Adaptive Form and this IS responsive and will bring over the scripting ect....  If you have one or two simple 5 ...

You may also want to consider moving to AEM Forms JEE and using formsets, This utilizes lazy loading to provide improved performance and memory footprint AEM 6.2 Forms Help | Form set in AEM Forms

There is a sling setting that you need to set you remove this issue.  I don't have the setting at my fingertips but will update the thread as soon as I locate it.

What version of the server software are you using?  I just rendered the form you supplied with AEM Forms JEE 6.1 FP1

Can you provide the form so I can look at it please?

You can use the browser developer console by pressing F12.  You can set breakpoints and debug directly in the browser.

An XDP is a file extension for a file created in AEM / LiveCycle Designer, it's based on an open source specification known as XFA

Thanks for the questions, 1)  The Flash interface has been deprecated as you point out but much of the capabilities that existed in that approach are now available using Adaptive Forms and/or Adaptive Documents and embraces newer technology such as responsive design for viewing on mobile phones.  PD...