Another suggestion is I've never built a keyboard but I've done weird LED arrays and one trick to pre bend all the leads the same way, involves a chunk of scrap wood and a bunch of nails (and maybe multiple forming steps/bending jigs). Much like a sheet metal worker, you'll rapidly discover the leads will spring back a little and you'll have to bend them to 100 degrees if you want a right angle. Its just scrap wood and nails, so play around a bit. You can do it by hand but a bunch of nails in a block of wood works better and is faster overall.
You were worried a bit about getting the diode polarity backwards. You have the source and you'd basically be inverting the bits you're polling with, see the gh60/matrix.c code. The system depends on the internal pull ups so you'd want to not enable the internal pull ups and solder in a couple external pull downs like 1K or so. Just saying it would be survivable and more fun than redoing all those diodes not that its necessarily a good idea.
(edited to add, on your next build, if you want to try something really strange, wire up all the "rows" with 500 or so ohm resistors making a massive resistor divider such that the pin in the middle is at 2.5 volts and 0 and 5 volts at the ends, and then wire up the other pins in parallel. So you hit the 1st of 10 and get an analog voltage of 0.5 volts, hit the 5th of 10 and the analogy voltage of 2.5 volts (for 5V anyway) and have the stereotypical onboard A/D microcontroller input sample it. Some MC are not very high res or stable. I imagine this would be an interesting key debouncing challenge involving painful latency, but it would be cool in an insane way)