The process you have reviewed seems fine but the problem is that you are working within a results page. The results page is designed to list all of the records from your results recordset. This makes use of a repeat region, I think this is the problem you are having on this page.
Instead of using the default DataAssist results page you should recreate the display area of the page. You should remove the repeat region that is there by default and add in your own form in it's place. In the form add in your checkbox and then add the label next to it from the bindings tab for the value you would like to display. Once it is all setup just highlight the label and the checkbox next to it and apply the repeat selection.
Double check it after you add in the repeat selection to ensure that the submit button is outside of the loop. Once the page is setup like this you should be able to add in your Delete Multiple server behavior and have it work without issue.