You have a lot of questions,

In a fixed tubesheet exchanger, there is a problem of differential thermal expansion between shells and tubes. Tubes may expand at different rate than the shell, creating pushing/pulling at the tubesheet.

Hence, the metal temperatures of tube and shell are very important. They should be as accurate as possible.

An expansion joint will reduce this thermal expansion problem.

A bellows exp. jt. is lot more flexible than the thick (flanged and flued) expansion jt.