I try to answer some of these.
Since you're talking of tie-rods, my answers are based on considering the joint as lateral expansion joint.
-A little gap is provided by most manufacturers to allow the rotation of the tie rods and therefore, the lateral movement.
-A small gap is always required. Also, the bellows' own expansion/contraction needs to be accommodated.
- Expansion joints can both take expansion and contraction.But then these have to axial expansion joints (i.e without tie rods).
Hope it brings some clarity.
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SJ