LeakProof measures gaps in geometry, which is how light leaks in Cycles. Shadow-map light bleeding in EEVEE is a different effect and is not measured.

Only if you press Seal The Leaks. The seals are one object in the collection LeakProof Seals, in the material of the wall beside each gap; Remove The Seals deletes them and nothing else, and Ctrl+Z undoes both.

No. A butt joint does not leak in Cycles, thick or thin. A gap does. LeakProof tells you which seams have one.

Parts that nearly touch with the room on both sides — a chair a millimetre above the floor — are gaps, but no outside light can come through them. They are listed for the record, not as leaks.

Gaps are then judged by the sky alone and there is no shot to prove on; the panel says so.