Hi Shawn,
Greetings from Syncfusion support!
Query #1: Is there a way to turn off expansion behavior and ui for certain accordion items?
Yes, you can customize the expansion behavior of the specified accordion item by using Expanded property.
Query #2: Accordion in a sidebar dock
We have checked your case by loading the Accordion in a sidebar dock and we could not noticed the reported issue at our end. Also, we have prepared a sample based on your requirement which can be downloaded from the below link.
Kindly try out with shared sample and get back to us, If you have any further queries
Query #3: Accordion without children still has a dropdown icon
It is not an issue and also this is the default behavior of Accordion component.
Regards
Alagumeena.K