@shr
Indeed, for inline pressure belanced bellows Dm = 0 when you use a simplified model, but you can also model these with 3 xjs and 8 rods. It will give the same results. A tee variant can be best modelled with 2 bellows and tie-rods and using the actual effective Dm imo.

@mav
Please get some facts straight for yourself. The actual question you're dealing with is:
"Does a pressure in a pipe give axial forces in restraints?"
Check shr's comments for the answer.
Once that's settled I think you'll have the answer for your question about pressure balanced bellows too.