that good find of the Huret spec sheet posted by repechage shows the threaded stud in the shift lever braze-on as being a thread in piece. have you tried applying a spanner to the flats on it to see if it unthreads?

with it out your shift lever options would open widely. stupid me, had been assuming all along that it was integral.

since you already have the Juy suicide front mech you could make up a vintage Juy gear ensemble to tide you over. visiting a bicycle co-op/charity might turn up a vintage metal rear and shift lever you could fit for the time being.

wrt chainset -

repechage's find puts the bicycle's birthday back from ~1959 to more like ~1954. this means she could have come with a 49A chainset. sadly, these are both scarce and dear...