Spiral Balance (non-tilt)
We don't sell any spiral balance with a 1/2" diameter, unfortunately. The Series 500 would be our closest at 9/16", but aluminum windows don't give you a lot of space for variance. I would triple check that detail just to be sure.
In terms of length, you can round up to the next inch. In your case 21" should be fine.
Finally, for the bottom bracket I would recommend the 17-012. Depending on hole placement it may be necessary to drill a new hole, but hopefully they line up.
Not at all, as long as at least one of the holes in the 17-012 match up with the hole in your sash then you will be fine just using that. Like you suggested, the extra holes were put in to accommodate many potential hole placements that are common on older windows. One screw should be perfectly fine.