Have you tried another operating system? I have used Cinnamon on an XPS 9360 with an external display hooked up via thunderbolt to displayport adapter. I had some of the issues you described, especially the sleep issues.
I asked about this because I'm not sure your issue is totally related to using a thunderbolt dock. Laptops generally require a lot of OEM developed drivers to fully function. It is very possible you would not experience these issues with Ubuntu which is fully supported by Dell on the XPS.
My understanding is that Mint is heavily based upon Ubuntu and assumed there might be none such differences. Thanks for the suggestion -- I will definitely investigate in that direction.
I asked about this because I'm not sure your issue is totally related to using a thunderbolt dock. Laptops generally require a lot of OEM developed drivers to fully function. It is very possible you would not experience these issues with Ubuntu which is fully supported by Dell on the XPS.