Well, I got it fixed. What a nightmare taking out and re-installing the toilet!! sheesh!! Its extremely difficult to access that rear nut. Some past mechanic even went to the trouble of cutting an access hole in the side of the toilet that helps...can't see it; the toilet is so close to the wall on the left side, I only found out it was there by feeling around. It is impossible for human arms to access that nut through the back of the toilet. Is there some trick to doing this? something I'm missing? getting both of those closet bolts into their slots AND through the holes in the toilet at the same time was a real bitch. (oh, and I'm aware of the access hole in the rim of the toilet; useless, as the height of the closet bolt and its proximity to the bowl makes it impossible to get a deep socket on to that nut. not enough clearance; anyway, that doesn't really impact the othe problem of getting the toilet back onto the bolts when you're done).
The valve replacement/re-plumbing went pretty well, though. I was able to find a series of fittings at HD that converted the female fitting on the ball valve to a 1/2" male. I wanted to use braided flex-pipe to turn the corner into the vanity to attach to the water supply, which was 3/8 copper. turns out that HD had the braided stuff, and it even had the 1/2 pipe thread connector on one end, and 3/8 compression on the other! (serendipity
). So I just cut the bent 3/8 copper supply line under the vanity, connected the compression fitting, connected the flex-tube to the compression fitting and the toilet on the other end, turned the water on and winced......nothing leaked! first try!! wooHOo!! could't believe that...something always leaks on the first attempt