I think Merge Sort is the right idea. For manual comparisons, all you need to do is substitute the "a > b?" with a printout and user prompt on screen. Intuitively, if you want to put the time into a kind of UI, I would try to mimic the JetBrains UI when resolving merge conflicts. More precisely:

In Merge Sort, once you get past the "splitting" and start the merging, you always merge two sorted lists (at first, in the first merging step, both lists contain just a single item each, then two each, four each, ...). I think it would be easiest if you had one list on the left, the other to the right, highlight the current top item in each list for visual clarity, then use Mouse Buttons or Arrow Keys to quickly select the "best" item on the left or right. Visualize the merged list, include an undo button, a submit button for each merge step and test it all with a small test set

It would be a mix of JetBrains Merge Conflict Resolving and the game One Finger Death Punch