Ahhh... I see.
I would suggest that you bring the cabin-top to its intended position for a proper repair job. The subsidence is due to the failure of the core and even though you would eliminate further deformation by making the repair where the pieces sit now, the best solution is to do it right. After all, once your commander is all shippy, you will not want to see that issue at the top of the post. It will not look right.

I imagine that if the cabintop does not pop back-up on its own, you can help prop it up into position fairly easily... Then do the repair as suggested. This is an important area, but it will not be hard to get it sorted out.

Your dermel-like tool may do the job, but it is a great big job for it... Not the best tool for this application. An Angle grinder with a cutting wheel, or even a skill saw with the proper blade would serve you best. Angle grinders are quite versatile. I'm sure you'll use it a lot if you cannot borrow one for the job, and end-up buying one...

BE careful with it though... They are powerful and can do lots of damage! (I am working on healing a nasty abrasion from one at the moment!)