sp_WhoIsActive waiting resource
This question is more about finding a "way" to figure out the waiting resources (using sp_WhoIsActive or similar) than actually fixing this current performance issue


If you're looking for what these numbers mean then, check out the sp_whoisactive Documentation.

From Adam Machanic's the page on the CXPACKET node identifier, you can see that this line in your output:


  (5x: 68/168850/635541ms)CXPACKET:15
  (3x: 3/16/32ms)CXPACKET:7
  (1x: 10207374ms)CXPACKET:1  


Means that there are 


5 tasks waiting on CXPACKET in node 15 of the execution plan
3 tasks waiting on CXPACKET in node 7 of the execution plan
5 tasks waiting on CXPACKET in node 1 of the execution plan


The 3 numbers separated by slashes are documented here:


  (Nx: MINms/AVGms/MAXms)[wait_type] where Nx is the number of tasks, MINms is the shortest wait duration of the tasks, AVGms is the average wait duration of the tasks, and MAXms is the longest wait duration of the tasks


The output of @get_task_info = 2 is telling you 


where exactly in the execution plan your query is getting stuck
how many tasks are getting stuck there, and
how long they're stuck there (min, max, and on average)


Hopefully this provides enough information for you to be able to find which part of your query is getting stuck on these parallelism waits so you can investigate further.