Kind of difficult to give recommendations on where to start for resoldering, but my first hunch would be cold solder joints somewhere. I have a Kyria from splitkb that I assembled myself that had spotty LEDs on one half which turned out to be a cold joint on one of the surface mount underglow LEDs. Also had no key presses registered on a row that turned out to be a cold joint at the MCU.

As for general troubleshooting recommendations, if you can get a board schematic that would be immensely beneficial for your efforts as it would show how and to what pins of your MCU everything is connected. With that you can try to identify where the fault might be occurring (e.g. LEDs die after LED 5 in the chain) and focus your efforts before/after that area.

Failing the board schematic, you may be able to just visually see where the traces connect back to on the PCB, or you could probe it out using continuity mode on a multimeter and reverse engineer the connections.

Another thing that may aid in diagnosing where the issue lies with the double key presses is figuring out how the key matrix is laid out. For example if you're receiving double presses on only some keys in a single row or column, the issue lies in either that row/column or the MCU pin they connect back to. Again, the board schematic would be really helpful in this regard.

Best of luck!