It sounds like the handbrake unit has died, but God only knows why it's done this soon after the battery was reconnected.
Pulling the release cable in the boot requires quite a strong haul to get it to release, but doing so renders the unit useless and it will require replacement.
Unfortunately these units are expensive ( £500 + ) and require reprogramming by someone with the correct software - mostly only dealerships, although some independant garages can do this - before they will work. Second hand units can be sourced but are a bit like the proverbial "hens teeth"
They are not difficult to replace so it's a job that can be completed by someone with a decent bit of mechanical knowledge.
I replaced mine last year and it only me took an hour or so. I got it programmed at an independant garage local to myself at 1/3 the cost the dealer wanted.
It may be worth seeing if you have a garage local to yourself who can check if the unit can be reset first. It could save you a lot of cash.
I'm sure I put a thread together with some guide pics if you need them. Yup, here it is ........ Scenic II handbrake module
Having re-read your original post it seems like only the passenger side is stuck. You may manage to physically pull the cable back enough to release the handbrake on that side of the car. If you do manage to free it off it might be worth attempting to get some WD40/release
in about the cable/lever on the caliper.
You should see the parts I'm on about in the pictures in the above linked thread.
Good luck !