Jacek Fedoryński

Data bits should be 8 (again, on both sides).

I don't think so.

Not for rev1.1, but there's a version of rev5: https://github.com/SkylaHila/flatbox-southpaw

I think you need 2mm pitch headers, not 2.54mm.

Yeah, looks like it. I think some of the holes are not used, but it won't hurt to solder them too.

I don't know, but surely the board came with some version of the firmware already flashed, right?

I think it's called "PCB Assembly" now.

These are the keyboard and tact switches, they're not assembled by JLCPCB, so just ignore it and continue. (They shouldn't be in the BOM file in the first place.)

I just meant I shouldn't have included those entries in that file. Use the same files and just click "continue".

No, use BOM and CPL files as you did and just click "continue".