Deadline + skinny pliers + coffee + WD40 (not recommended)
- Deadline: having guests tomorrow and need to make room, so these shelves gotta go up today!
- Skinny pliers: run to a local tool store and find a cheap pair of skinny-nosed "snap-ring pliers" that will fit in those little gaps due to the insert being intentionally not-round
- Coffee: essential ingredient in any DIY projects (mine at least, decades have taught me that it works better than beer)
- WD40 or equivalent: by trial-and-error I found that this is more effective in between the metal surfaces than in the coffee.
The springy metal insert is quite soft, so the small gap was all that was necessary to get the pliers in and work them down.
But what actually released the insert was pulling and twisting at the same time, compressing the spring by pulling one end of its "C" shape towards the other.
Release was sudden - exercise (more) caution (than I did).
Since one of the insert was somewhat damaged/compromised I'll be careful not to overload the upper half of the shelves, and toss the whole thing next time I move.