I tried running the hot wire from the jack to the outer lug on the volume pot instead of the middle, which bypassed the volume pot. From this I gathered that the crackling noise was because of the volume pot being faulty for whatever reason, my reasoning being that if it was a grounding issue, it should still have been crackling if I bypassed the internal circuit of the pot. So I replaced the volume pot with my last BKP pot (bought 3 for this exact reason), I was very careful not to apply excessive heat, and it doesn't do the crackling at all anymore! Yay 8). Unfortunately I wired something wrong, so the volume pot now acts weird, which is the next thing I have to fix.. Hehe :lol:
Though the guitar is generally still a bit noisy, I will try adding a ground bridge to see if that fixes the noise. The guitar did have some kind of extra bridge grounding mechanism but I couldn't find any good info about it so I took it out. Something like the ground wire of the bridge pickup was connected back to a little metal ring wedged under the bridge plate itself by the screws. I read that the screw grounding is enough for some, but others don't trust it and add the second ground via the wire you suggested, so I have to try it.
Something that really confused me was that in the original guitar electronics, I saw a ground wire was lead from one of the pots to a little metal ring in the cavity, for no obvious reason. Just one wire out, soldered to a piece of metal that didn't go anywhere at all, just screwed into the wood. Any hints? :D