I can't answer all your questions, but here's what I know:
Will it impart any flavor to my starter?
The juice is just added at the beginning, I assume. As you repeatedly throw out half of your starter and re-feed, you will quickly get to the point where the amount of juice is infinitesmally small. It would be fair to say that after 6 or 7 feedings you will have no juice left in your starter, and therefore no flavor from the juice.
most all recipes I've seen call for whole grain whole wheat or rye flour. I have seen others that have used white flour. I'm looking for a more subtle, non-commercial taste than ADY but not a strong sour flavor. Which would be best for me?
From everything I've learned rye flour is only used at the beginning. Afterwards you can use whatever flour you want.
I've baked in my kitchen with commercial ADY for months and months now, many times a week. If I set out a starter, isn't there a chance I could just capture the commercial ADY strain?
Nope. The reason starters can be fead indefinitely is that there is a symbiotic relationship between the yeast and the lactobacili; they help each other live. ADY is only yeast, so you cannot create a commercial yeast and feed it; it will just die. There is no chance that you will pick up the ADY from the air.
Incidentally, yeast is what gives the puff, and lactobacili is what gives the flavor.
Some instructions say to discard ½ of the starter every 24 hours or so (after the initial 48 hours). Some skip this step. How critical is this? Should I do it?
It won't make a difference to your starter. The reason most say to throw out half is just for the shear amount of yeast you will end up with, and the huge amounts of flour it will take to feed it.
With all this said, I would really suggest that you buy a known good starter, or get some from a friend that already has some. To use Jeff Varasano's example: if you set a trap in the woods, you might catch a moose, but you might instead catch a rat. You might end up with a starter that is too sour, doesn't have any flavor, or doesn't give much puff. Known good starters are dirt-cheep, and trying to catch your own strain can be more trouble than its worth.