Ah, so that probably explains what those scratches along the groove were about. They match perfectly with the use of a wire tool.

Glad it worked out. Thanks for the update!

As kitwashere suggested, did you remove the edge plate and look for 1 or 2 small holes just to one side or both sides of the bolt (the holes would be covered by that plate)? The hole(s), if there, would be very small and hard to notice unless you knew to look for anything there. I think a small hex driver (perhaps ~.063", .078", 2mm...) is typically used to back out the setscrew(s).

Remove the faceplate on the edge of the door (at the bolt). On one or both sides of the bolt, look for a small hole. If present, there may be setscrews in those holes that I believe engage the slot you mentioned. If so, back out the setscrew(s) and then the cylinder should unscrew from the internal lock body. Good luck!

If you find it detecting metal intermittently along a straight vertical (same vertical line as the "stud" setting), those are likely metal drywall screws. If the device senses both "stud" & "metal" continuously, then I would expect there are metal studs, which is common in modern construction of larger buildings.

You may be getting a into electronics here. LEDs often require a driver circuit to provide the correct voltage and current. If the system was sized for a certain voltage drop and current draw, and the new LEDs are requiring something else, the power supply may not function properly.

Unfortunately, I am no expert in this so I don't think I can offer much more (my work with LEDs is limited to relatively simple circuits and repair of a couple LED desk lamps...but I was able to educate myself through various internet sources just enough to solve my particular issue).