Typically components added to an editable template cannot be deleted by an author. Do not put in components into a responsive grid at the editable template level. Should be placed in the page level to let authors add/delete a component.

You can also remove auth requirements so your 3rd party app can successfully invoke it.

I am not aware of any way to customize the widgets available with a content fragment model. I will check internally to see if anyone has thoughts on this.

I am not aware of any other way.

Is this API what you are referring to? "The Adobe AEM Quickstart and Web Application."

For reference - so community can try and reproduce - please point to the AEM docs you are following for this use case.

For AEM 6.3 - you are using static templates. Sounds like something is wrong with the static template. See if you can get the AEM 6.3 Toy Store (which uses static templates) working: Scott's Digital Community: Creating your First Adobe Experience Manager 6.3 website

See this article which works fine on 6.4. To convert to AEM 6.3 - use the AEM 6.3 UBER JAR file as opposed to the 6.4 one. Creating an Event Listener for Adobe Experience Manager 6.4

This is not new - been around since 2014. We have plenty of docs on this - see this: Adobe Experience Manager Help | Querying Adobe Experience Manager 6.4 JCR data using the QueryBuilder API

This looks like an upgrade bug - this will not happen on a fresh instance of AEM. Please open a support ticket,