Ok, there are a few things troubling me about this setup that I haven't been able to figure out. As far as I can tell, a blend pot looks like it's just two pots on the same shaft, whose sweeps travel in opposite directions, so that both volumes are all the way up when the knob is at its center position. That's why the six lugs. What I can't figure out is how to wire that in where it won't just be redudant with your other volume controls. You could put it in between the pickups and the selector switch:
and I think it would function as it's supposed to when both pickups are on. However, it would also affect the sounds of each pickup on its individual setting. For example, if you had the blend knob turned entirely in favor of the bridge pickup, when you switched to the neck, you'd get no sound. Is this how it's supposed to work? I've never owned a Ric (real or cloned) so I don't know if that's the intended function. That seems redundant with the existing volume controls to me. I can see having it instead of a selector switch, or in combination with a master volume instead of individual controls, but in this configuration it doesn't make any sense to me.
Furthermore, I don't see in the Ric diagram where they're using a blend pot at all. The thing they're calling "mixer" doesn't have enough wires going to it. It looks like a regular pot that sends some of the output from the neck pickup past the tone control.
By the way, the bare wire coming out of the pickup wire is a shield, which you'll want to connect to ground if you haven't already. Shielding is used to combat hum and keep things like nearby radio stations from coming in through your guitar.
If you've got scratchy sounds when you turn the knobs, yes, replacing the pots should take care of that.
I've never adjusted polepieces on my guitars, but I'm inclined to agree that it won't make a big difference. You can always turn them a little and see if you can hear a difference. The bottom line is if you can't hear it, it doesn't matter. You can get a more noticeable effect by adjusting the height of the pickup. If you want to lessen the bass response, you can pull the bass side of the pickup back away from the strings.