I am making progress. I think the code on MDL-32188 is basically done, and I plan to test it a bit more tomrrow, and then submit it for integration this week, so that it goes in to Moodle 2.6.
Note that this is a major change to how CBM questions work, but I hope that is an major postive change. (Thanks mainly to Tony for pushing me to do this.)
The real difference is that now when you add a CBM question to a quiz with max mark 1, then the grade for a low certainty response will be between 0 and 1, and the grade for a high certainty response will be between -6 and 3.
If you have exiting attempts at a CBM quiz, then they will not change until you regrade them. However, once you regrade them, the marks will change. Simiarly all quiz attempts done after you upgrade to 2.6 will behave in the new way. You have been warned. Assuming this gets into Moodle 2.6, we must remember to add a bit warning about that to the release notes.