Hi Matt,
All your discoveries are bang on.
As you've seen - a .580 grip on a .600 butt makes for a 1/64 oversize grip (the .600 butt stretches out the grip core). Two layers of buildup tape (plus one layer of 2 sided tape) with a .600 grip and a .600 butt
also makes for a 1/64 oversize grip.
This is the same thing that happens with multiple layers of buildup tape and core sizes that match butt sizes. I don't know the thickness of double sided tape, but for buildup layers, you only need to use masking tape.
http://www.tourgolfproducts.com/WORKSHOPS/AW_2.asp
That's a beauty chart showing how many layers of buildup tape are needed to make certain grip sizes.
The other option for oversize grips is to buy oversize models (often marked 'mid'). These grips have standard cores (ie .600), but the grip is thicker, simulating buildup tape.
Now, to answer your questions:
1) .600 at the butt.
2) .600 grips with one layer of 2 sided tape makes for a standard size grip.
3) Honestly, this is all feel. I myself prefer smaller (standard) grips (even though static charts put me in a +1/32" grip)
4) By all means, YES, use double sided tape. You need to know if it's water or solvent activated. You'll have to be generous with the solvent to remove the grips with air (but it's still quite do-able).