I think writing your own ReportExplorer is your best option in the long run as you'll have the most control over things. Handling drag & drop used to be possible in a much older version of AR. See this article for the details in how it used to work. You might try that approach with this version (i.e. handling DragOver event yourself).
If those events aren't working and Drag & Drop is really important, contact ComponentOne support and see if they will give you the information about exactly what drag & drop data you need to put in the drag event to get the design surface to respond. And/or ask them if they will reinstate the behavior in the article above.
Another option is to add some buttons to your application like "Insert Textbox" and when the user clicks on it, you programmatically add the item to the report. To us geeks this might seem crude at first, but in my experience drag & drop is not all that intuitive for end users so it might be a perfectly acceptable option. That depends on your users though and you know better which approach to take.