Currently Working with MK to resolve. It seems some bunch of boards had bad QC on them. Another user on the forum is also having a different set of problems!
My shine 5 has the same super small movement in the backspace just like you describe and it's 100% perfectly fine.
I think your problem isn't the switch. It's probably the stabilizers.
A lot of keys on my shine 5 have micromm movement and they all work just fine.
I just tested my shine 5 for you.
Backspace: small movement.
Numpad+: small movement
Numpad enter: small movement.
numpad minus (no movement)
L key: very small movement.
You're afraid of ghosts.
No Not ghosts and I am afraid that statement is unfair.
I bought two shine 5s. I love the board. I wasn't in anyway down playing how AMAZING this board is. How ever. Let's make a clear distinction. I am not saying micro movement in the caps. I am taking about the switch it self. Remove the cap and the switch moves in it's place. Nothing to do with the stabilizer because the backspace on one board is perfectly fine and the switch is firm in it's place.
You are correct in saying it works fine, BUT, it sounds really weird. I compare the two boards and it sounds bad. also since I use my pinky to press the backspace often in the leftest most part of the key and not the center it sometimes doesn't register. This isn't an issue on the other board.
There is also a number of quality defects I have, I attached some proof. Buttons are not cut properly. the row with the volume button isn't even aligned at the center. Does it work? Absolutely? but if I pay a premium for a board I expect premium quality.
Now ask me would I recommend this board ? YES.
How much would I rate it? 11/10
Why? Because I have two (one is for a sister/friend) and I am an honest guy. I opened mine and used it. I din't swap it to her when I i realized hers is 100% on QC while mine wasn't. Life is such and sometimes you get a bad pick. Now I am sure Ducky and MK, who both have excellent service would be willing to replace the board. BUT, the RMA cost of shipping for me doesnt justify my 5% loss of satisfaction for using the board. As you clearly mention the board is 100% functional.
It is a principal, for me. When I pay for something I expect my money's worth. I think I got my money's worth. It is my first MK and i love it with all that I have. It's not perfect but its mine

However even though I believe in ghosts, not afraid of them at all.