I have considered a nonlinear analysis due to the fact that physical contacts will be involved. Having physical contacts allow me to add fictitious clearance which gives the same effect as the bolt has been tightened. I also considered some frictional effect, more with sealing and less between casing and bolts. The symmetry boundary conditions were also applied due to symmetric model taken in to consideration.
Yes, physical contacts were required here due to fictitious clearance.
Of course this thing you can do in this case also. You just need to output the forces on either the bolt or case contacting face. This will give you the force required to compress the sealing that much.
I hope I answered the questions. If not, please feel free to ask again.