I've got the first ball knit, and it does look great.
Some doubts arose, however, in the middle of the night. (I'll blame Nigella's Bread-Sauce Scented Potato Gratin. Which is, by the way, delicious, if difficult to explain to someone who wasn't brought up in the UK.)
I'll relieved to report that in the harsh light of morning my panic about the cables was for nought. It had only just occurred to me I would have to possibly change the cable twist direction once I was "over the top" and heading down the back. And then I was concerned that because I'd be grafting together cables that had been worked in opposite directions, they might not look the same. The good news is that you can't tell the difference, and I wouldn't even need to change the direction of the cable twist.
I'm now not sure about the colour progression, though.
I did the math yesterday on when I will need to introduce a new ball and force a colour change. The body (up to the armhole break) will take just shy of four balls, and the sleeve portion just shy of 6. So here's the thing. I've got 2 each of 5 different colourways that form a lovely 5-ball colour progression.
The original plan had been that I work them in order 1 2, 3, 4, 5 up to the neck and then 5, 4, 3, 2, 1 back down. With that, each colour from each ball will appear precisely once on the front and the back. (If you recall I decided not to do that to eliminate the seams and possible non-matching stripes on the sides.) (Also recall that the colours of Kureyon I've chosen have non-repeating colourways -- that is, each colour appears in each ball precisely once.)
To use the balls I have in the circular variation, I'm going to have to do it as follows: 1, 1, 2, 2, up to the armhole break, then 3, 4, 5, 5, 4, 3 up and over the neck to the back. But this means that colours from colourways 1 and 2 will repeat.
The alternative is to rip it out and go back to the original design, and live with the possible mismatched stripes at the side seams. I'll be able to control this relatively well in that I'll be able to match up the number of rows from each ball and therefore be lining up the same colourway, at least.
Maybe I should just relax, huh?