Please check the somExpression of fields if they are resolving properly. This may be the reason or you can share your form @ (vkatoch07@gmail.com), I'll have a look.

Your Adobe ID should be linked with your Adobe Experience Cloud solution accounts to download the installer. When you bought it, Adobe must have sent you an email or some link to download it.

XDP files are specifically designed for Adobe's form creation and merging them with the XML to create the PDF document can be done with the AEM services either directly or using their API programmatically. 

I haven’t done this, but by creating a custom code it can return the list of adaptive forms from the forms content path or if the forms are listed on the forms portal, from there I think we can also retrieve the list of forms.  https://experienceleague.adobe.com/docs/experience-manager-65/forms/publ...

You can give left, right, top, and bottom margins in the subforms as per your need.

See the images of how I changed the highlighted to flowed and wrapped the positioned subform inside the flowable subform and it worked.  

Have you made the Main page flowable and allowed pagebreak in all flowable subforms and on the main page?   If it still doesn't work, share your form. I'll look into it.

Can you please check if you are missing the mentioned bundle in your AEM instance or using the old version incompatible with the code? Please verify the bundle and update the version or code accordingly as per your environment.

Yes, that's the case with the positioned layout. You can do one thing if you do not want to change every subform to a flow layout, then let it be positioned and wrap them under a subform with a flowable layout.

Yes, it will jump because you have made others as positioned. So make other flow as well. It will be good for future changes, if you need to add/delete any fields in the future then the flow layout is best, it adjusts the form itself.