I usually make my decisions based on the meaning of the expressions rather than solely on the font and size of the print. Here are my thoughts, and I mostly agree with what you've laid out -
I can't see measure 1, so I'm not sure what the expression is (but you're more than likely correct in using it as a music heading).
M 6 Cedez – relax the tempo (in the music line)
M 8 En mesure – in time (in the music line)
M 10 un peu retenu – holding back a little (in the music line)
M 11 En elargissant – widening (in the music line)
M 12 1er mouvt – first tempo (free line, music heading, applying to the second half of the measure, of course)
M 13 Tres lointain – very distant (free line, music heading)
What do you think?
A lot of this is transcriber's choice, but this is what I think I'd do here.
Hope that helps!