Here is Javadoc. https://sling.apache.org/apidocs/sling6/org/apache/sling/api/SlingConstants.html#PROPERTY_PATH. What version are  you referring to?

Sounds like a bug, please file a ticket so support snd Eng teams are aware of this issue.

That topic talks about managing users, groups, and permissions. For example, how to set permissions or create groups.If you want to see the experience that a specific uset sees based on permissions, etc, then log into AEM as a specifc user.  Tjis is how people tests users, groups snd permissions.

File a ticket and state feature request,

Are you trying to access servet using Ajax fom AEM JSP?

Not clear on your question. Why not use multifield?

What Java classes are you trying to use. I will test this to see if i get same results, Let us know the Java classes and the POM depedencies you are using. 

In your modification, did you remove code that lets components to be dropped? 

Read these AEM Sightly Articles - there is a lot of code that shows this: https://helpx.adobe.com/experience-manager/using/movie.htmlhttps://helpx.adobe.com/experience-manager/using/aem_headline.html

This requirement can be solved using JQuery: https://www.formget.com/jquery-registration-form/You can easily use JQuery with AEM. See: https://helpx.adobe.com/experience-manager/using/integrating-jquery-framework-cq.html