Sven,
It most definitely sounds like a dead battery.
Can you put a trickle charger on it and leave it for a few hours? Or is their no power sockets near the car?
As an alternative, jump start it from another vehicle (obviously with the engine running).
If the car was put away wet, your deduction regarding the brakes is almost certainly the correct one - a little spirited braking should sort that out.
no rear-towing point on my Roadster,
had one "made/fixed" for it
imho: jumpstarting
and then the brake-pads "easely" but "noize" unstick from the disk
