New here to Shimano chains and 12 speed. Always used KMC 10speed chains with reusable quick links included. New bicycle was delivered with RD B screw way out of spec and resulted in me having to cut the chain. This means I had to open the Shimano SM-CN910 12 speed quick link, cut 2 links out and reinstall the quick link. Then, DOH, accidentally installed the quick link with arrows facing the wrong direction. In effect, I reinstalled the quick link twice

Much debate on this as Shimano says only one remove/replace per link. IME if it is easy to open with your fingers than probably time to replace. If there is still resistance to get it to snap onto the pins, I would use it. I've never had a QL fail in over 100k miles of riding with them from mostly KMC, but a Connex and SRAM thrown in there too and I open/close them usually 30+ times before replacing them when the pins start to show wear inside the links. Connex makes a 12sp that's OK for multiple times but around $30+ from reputable sellers. Cheap fakes of questionable quality are available for less on Amazon, Alibaba,etc.

My rule for Quick Links, if they are tough to open, they are tough to open.

I’ve a keen sense of the obvious.

Barry

So it appears you fixed it. Having to re-install the quick link because it was backwards is par for the course in DIY. And is just extra tuition fee for the school of DIY.