After you check the lock, make sure that you try both sides of the folding seat.
If you can get inside, you should be able to easilly remove the trunk lock.

If the seats are locked, I checked my car and a six-foot man can easilly reach in and unlock the driver's side rear seat.

If neither route is available, go to your local fire department and ask them if they have a "Rabbit Tool" or some similar hydraulic device to force doors.

If you can get inside, I'd try to force it from there, as there is probably enough room to use a larger tool if need be. I think that many "jaws of life" will fit in the trunk. (They would probably force the trunk even if you welded several inches of it.

If you cannot get inside, ask them to force the trunk with a hydraulic tool right in the middle. Try to pad the tool to minimize damage. Most car trunks that I've tried at extrication drills have opened fairly easilly. Surprisingly, many will shut again, showing little damage.

The 850 trunk looks quite strong at its bottom, it appears to be a box section instead of a pressed lip.
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2 8s & 2 7s 700,000 miles total