extend ignore 8 7 11 extend ignore 6 7 12

should save the situation. Am I missing something?

This is basically what I also thought; you need to supply three stations to control it. I was going to suggest the opposite notation: extend prefer 8 7 12 meaning "to get to 12, prefer to start from the 8-7 leg". But either preferring one triplet, or preventing the other, should work. So I would be just as happy with your suggestion. Either of these would also solve the "extend stop" problem at the same time.