We know from Proven Guilty that using LC does NOT make a tracking spell work. It simply makes a workable tracking spell more useful (remember Harry needed more than Baby Hair?). And there is no proof that a headache makes a suppression by Mab or that it made him forget Mouse. So, my assertion is that making Harry not use LC was like a null...it made him use something that would not work.
You're missing the point. We have to assume that Harry would have used LC because
he has done it before and
he is satisfied enough with how well it works that he keeps going back to it. Would have, that is, if he had remembered to do so.
It's not for us to judge how effective LC is. If Harry thought it was a waste of time he would have dismantled it long time ago. Obviously he thinks it's worthwhile to keep and to use.
And it is fairly clear that someone didn't want Harry to use Mouse either. Tie those two together, along with, admittedly circumstantial evidence, and it clearly points to someone tampering with Harry in some way.
We Know that Mab has the power and the opportunity to do so. But, if it wasn't Mab we'd have to ask who else coudl it have been?
If it was just LC, or Just Mouse alone it wouldn't be so significant.
If it was important enough in the Plot for Harry to have his memory if it tampered with about it, then it's no where near Null.