make sure you jack the car up correctly of you can damage the chassis--
2cv,s need some specialist tools to do certain jobs
maybye buy a good workshop manual

ayjay,
isn't it a bit late to worry about damaging the chassis, considering that one of the front legs has come adrift?

As far freeing off a 'sticky' rear brake, get that corner off the ground, then a combination of striking the backplate around its edge plus tapping the drum with a mallet will usually free things off.
Even in severe cases, such as when a car has been standing on damp grass for years, this usually breaks the grip of rust binding the shoes to the drum.

ken

Your chassis problem could have been worse - this was mine - just as the squirrels had left it. Chassis swap, so rewarding and just 'simple' engineering. Make sure you get your chassis number etched/engraved onto the new one before you take delivery. Think brake pipes and fuel pipe replacement.
Good luck
Bob