I was playing around with the code and only issue seems to be that esc_attr($_GET['wpv_column_sort_id']) returns something like "types-field-gpa" and the sort seems to prefer "gpa". It also looks like, until this ticket gets processed (https://core.trac.wordpress.org/ticket/17065), it isn't possible to have one column sort ASC, and the secondary sort go to DESC – not that big a deal.
Regarding the first item, does that make sense? Is there an easy way to chop off the 'types-field' part of the Get output?
Thank you for everything!