A complementary Sudoku problem

Inspired by the original T. Snyder’s “What If” article, I tried my hand at constructing a Sudoku problem whose difficulty increased as much as possible when a single element is removed, while retaining human solvability. This resulted in this pair of puzzles, one of which can be used as an introduction to Sudoku, while its near twin requires considerable experience to complete in a logical manner. Scanraid’s grader assigns them ratings of Gentle(27) and Tough (151), which concurs with my gut.
The puzzle below is the difficult one, while the easy version, and some spoilery commentary follows after the break.

Massively Happy

This is a mystery puzzle in video form. By mystery puzzle, I mean a problem in which the player infers its rules from the activity itself, and its answer is (usually) in the form of a word, or short phrase. Mystery puzzles are typically “scored” on the basis of time you take to solve them; in this one, you can also choose to solve with less information, and use observation in lieu of logic, or vice versa. Please do not spoil the solution in the comments section.

Inaugural prayer


Sandpaper Simians
I shall endeavour to bring my posts to a high sheen before uploading, and let the Three Wise Craftsmen be my moral compass.