Sorry Pierre, I have been ignoring your question because I don't have a good answer right now. There must be useful ways you can help, but I am failing to think of good ways to split up the work. Also, I am currently swamped with 'help': http://tjhunt.blogspot.com/2011/04/performance-and-scalability.html.
I think right now it may actually more helpful if you don't try to help me with the new question engine. I may be being a bit too over-protective of 'my' pet project, but there are still some parts of the work that I would really like to do myself - if only I could get the time.
In the mean time, it is really helpful if you help people out in this forum, and work on fixing Moodle 2.0 quiz and questions issues (even though I have not yet found time to review your fix for MDL-24594). Actually, a version of that patch that works with the new question engine would be really good.
I will go on thinking about how best you can help on the new question engine, because your offer of help is appreciated.
To answer your question about the new numerical question type we are creating. You are right that we don't use the Moodle calculated question type at the moment. Most of our questions like that are done in our other OpenMark assessment system. So what we are doing is making a Moodle question type that does what our users are familiar with there.
On the one hand, that is just making a mess for the future, because it does not really make sense for Moodle to have two different calculated question types, and so at some point we might have to try to merge the two.
On the other hand, I look on this as an interesting opportunity to try some different things in relation to calculated questions, and in the future we may be able to create a single calculated qtype that combines the best of both.