Thank you for contacting Syncfusion support“Query #1: Is it possible to template the selected items displayed on top to have a tokens combobox ?”
We would like to inform you that we don’t have this support “Close the Individual Items in ComboBoxAdv” currently. So please create new incident under your account to track the status of this feature. “Query #2: is it possible the style the checkboxes and scroller of the popup window?
We have prepared a sample which tries to meet your requirement. In the sample we have overrided the style using Resource Dictionary and we have changed the background color for check box and scrollviewer. Similarly you can override the style as you desired
Please download the sample from the below link
Please let us know if you have any concern