Drop 0.7V using a diode for use of a lithium battery with a 3V mouse?
I could theoretically drop 0.7V using a diode


This voltage drop depends on the current through the diode. A wireless mouse has very close to zero current flow when it is in "sleep" mode - resulting in a significant voltage rise at the output.

I recommend just using 2 sets of NiMh rechargables - the type with low self-discharge rate.