To question 2,
Setting an index to "NULL" as is in the image you have shown is to "relinquish" the set value. You can do this on the data point details page, and it is also possible to do via script.
To question 1, I am not sure what you mean the template for the priority array. You should be able to read the priority array into Mango, but it may require some additional processing to get it from a string representation to whatever representation would be suited to your use