you need to test this before diving in and learning the hard way like i did
if you use the m10 volume to adjust the headphone jack thats cool except there is no way to know if the level is still to hot or low
if you can record the opening act then playback some of the tape and see if you brickwall/clip you will be able to know if you are to high
and you shouldn't have to worry about drift because the camera is capturing the audio from the recorder directly
with all that said, i have a 20db attenuator i use now and it worked perfect with my canon and fr2le
i have sold the fr2le and not tried the m10 yet but i am much more confident its going to be fine vs adjusting the headphone volume