I don't doubt they will not want to make more of a commitment than they already have although considering they have already clearly stated that "This error only occurred in a small volume of models during 2013/2014 models" then they really have nothing much to hide if this is truly what they believe. Especially when they say that it's preventable through a software update. It's almost a non-issue according to them so what could they possibly be concerned about making a clearer commitment? They've already made a commitment. We would just like it to be better written and less ambiguous right?I don't think they will care, based on experience, but feel free to pursue this.
What leverage do you have to convince them to sign such commitment anyway? If you think they care about customer dissatisfaction, or repeat business, then you may be mistaken...
As I haven't had the issue and you are clearly hacked off about having two motors replaced, then why don't you construct a letter and we'll sign it? I would happily do it if I was actually affected by the issue (so far!) but it would just come across as 'unnecessarily moany customer' to them for all the right reasons.