Thanks for that @pwood999 - you've made it much clearer what my issues are. I suspect that whatever traditional tool is intended for use, it will no longer work on rusted and rounded heatshield bolts / nuts to get them off. I think I may be spending some time covered in crud while carefully applying a Dremel or multitool blade to cut the old bolts.
I will try mole grips on the central ones first as they look most critical for security. I am sure I can invent an alternative, preferably non-seizing method of securing the peripheral points for reassembly / any future disassembly. If so I will of course share it here for everyone's benefit.
From your photos it looks as though manifold removal is necessary to get to the head bolts too. Not just a case of sliding / easing the heads up past their joint with the manifold. I presume that with the manifolds off it is necessary / best practice to support the exhausts / cats in some manner to avoid strain on the system?
I seem to recall MrGorsky just chiselled them off in the end and used heatwrap on reassembly.