Based on seeing version numbers (rather than dates), I'd guess you're importing a style that's already out of date. As such, your templates are effectively last edited when you do the import so it doesn't have the version to compare against. Merging is not designed to be used with importing an old style; it's designed to be used with upgrades. (The system doesn't use version numbers for comparison in any way; it uses relative edit dates which allows merging to happen between parent and child styles for example.)
No, as you need to know what the older version of the template is as well as the newer version.
A template is out of date if child.last_edit_date < parent.last_edit_date. We need to know what version of the parent the child version is based on; this is the most recent where parent_history.edit_date < child.last_edit_date. The history (old), parent (new), and child (yours) form the 3 things needed to do the merge.
When you edit the child, it's assumed that you're bringing in the necessary changes from the parent. As such, 3 way merging is no longer possible.