As copper is very malleable, grinding is probably not going to work very well. Sanding is better, however it can also leave copper fragments that could pose a problem for you. My best guess is some amount of etchant + sanding will be your best option.
I'd also seriously consider the total labor costs of your mods compared to making (or having someone else make it for you) your own pcb. Depending on the complexity & component costs, that may not be a feasible option--especially for only 150 units.
Best regards
Sticky residue is normally the flux. IPA doesn't dissolve flux, it just makes it runny. So if you gently wipe it off, all you are doing is smearing flux everywhere. Once the IPA evaporates, the flux turns into a horrible sticky mess.
The solution is to add a bit more IPA to the board, wait a minute or so for it to soften the flux, and then using something like kitchen roll, wipe in small circles, changing which bit of the kitchen roll you are using as you do (so that you don't smear what's already been wiped up back onto the board).
For intricate places around ICs, you can use the same process, but rather than wiping the kitchen roll on the board, use something like a pair of tweezers to push the roll down on the board and move back and forth around and between pins.
You should find the areas that have cleaned well stop being tacky. If you find anywhere that is still a bit sticky, then simply repeat the process.
What is also worth doing before you start cleaning with IPA is to gently chip off any really large blobs of flux - you can usually do this with a blunt instrument and as long as you aren't jamming something into the board it won't do any damage to the solder mask. The advantage of this is it means less flux on the board that has to be wiped off after it has softened up, so less that will smear around everywhere.
I've used OSH Park before, and never had an issue getting the boards clean with IPA and some elbow grease. As to why your particular board is being more stubborn to clean, it happens from time to time. Maybe you had ended up with more flux on the board, or when you were wiping off the IPA you used something that wasn't doing as good of a job of absorbing the grot. Could be many factors.
Best Answer
Industrially, stuff called "conformal coating" is applied to PCBs for a variety of reasons, but it will keep moisture out and generally massively extend the time over which oxidation residue forms.
For hobbyists, clear nail polish works, though you might want to be careful so as not to allow the acetone in the nail polish to dissolve the silkscreen patterns on the PCB.
If the oxidation is not causing functional problems, you might just let it go.