Hi,
Thanks for your feedback.
I think in the dot notation, you can simply set:
Advanced attribute key: admin_columns.position
Advanced attribute value: before name
(no '.0' in the key).
Using .0 makes the plugin think it's a multi-dimensional array. Please try again with that.
Regarding the sorting with value from custom table, we're working on that. At the moment search and sort for custom table is not supported.