Just finished building my Key65 and wanted to share my findings with using split backspace. My PCB also had the issue with the pads on the front and back sides not connecting. I was able to fix this using a mill-max socket on the pad. With the use of the dampening pad, you can't even tell that one side of switch is in a mill-max socket since the dampening pad is so thick.
Hope this helps anyone experiencing the split backspace issue. (Please ignore the poor choice of keycaps. This is temporary until Camping R2 ships...)
I know it's been a while, but I just wanted to thank you for posting this. I built my Key65 a couple weeks ago and it turns out I also got a bum PCB. I tried the factory suggested method of just jamming a buttload of solder into the socket, but that didn't feel good and in the end - didn't work. I also tried contacting Senter, but he hasn't responded yet.
I was pretty bummed out about the whole build when I found your post, but it gave me some hope of actually salvaging it. I ordered some mill max sockets and they just got here today. I had never worked with them before, so that was also a learning experience. In the end, I soldered two for the switch in question, just to be safe and diligent (and hey, now I'll have one hot-swappable switch on the board).
All I can say is that it worked! The key was immediately picked up in the VIA keytester. I'm so happy now. Not sure if you'll read this, but thank you for sharing your method.