Re: Alternator wiring
You didn't mention what year your car is, but I'll go with trying to answer your questions about the diagram you posted and some other questions.
Basically, the blue and white wires ran from the external voltage regulator over to the old externally regulated alternator and provided the voltage drop/distance through the wiring so that the regulator could see what was being produced farther away from the alternator. So, the long run of white wire that gets connected to the red wire provides some resistance. You could jumper that straight to the output of the alternator, but then you wouldn't get much voltage drop sensed, and the alternator would constantly put out lower volts. This is why you see some people jump it that way, and this is also effectively how the single wire alternators work (they jump it internally.)
In your steps, I'm not sure why you need to tee a new wire off the solder joints you are recommending - the blue and white wires should already be terminated somewhere near your alternator, so just connect them into the two terminals at the alternator and you should be good to go. If you tee off, you'll have to find the old two wires and cap them off anyway and will just have extra wires laying around.
BTW, this is the exact same mess I'm cleaning up in my car (72) right now - the PO put in a single wire alternator, which constantly only runs at about 13.6 volts (senses no voltage drop) and the warning light is always on because it has been bypassed (the brown wire.) I'm switching over to an internally regulated alternator, removing the voltage regulator and jumping it exactly as this picture shows.
1972 Chevelle, 454, 2004r, 4.10
1981 Camaro Z28, 355, TH350, 3.73