in reference to your points:
1. any deformation on elastic materials can be decomposed in volumetric (volume changing) and distorsional (shape changing, shear) deformation. By saying that rubber is incompressible, it is certainly not meant that it is rigid in compression: certainly steel is stiffer in compression than rubber. What is meant by that is that the volumetric stiffness is so much bigger than the distorsional one, that practically the first can often be neglected. In this sense, rubber is a liquid. Of course one can use the term shear strain when referring to how the angles between material lines change: in this sense a tensile deformation seems shear-less. But this definition depends on your reference system: that is why for example the pure shear test piece, essentially a very wide tensile tests, get its name from. The topic is addressed in much more rigour in many excellent books.
2. I understand better now. It is difficult to help. You are certainly right in that sensitivities analysis might help. The problem with those very high compression is, as I said, that a lot of assumptions might get too inaccurate: for example, hydrostatic and shearing stress might not decouple like commonly assumed. Or maybe, the very small viscoelastic relaxation in compression becomes not negligible. The stiffer a rubber is, the bigger impact little errors might have. I understand the test you performed: I would still perform the compressive test in fully confined, constrained set-up, not so sure if it covered by any Standard but would clearly tell you, by comparison with the test you performed, how well compression is captured. You can test and model both the set-ups, lubricated and laterally constrained, and compare. The bonded compression test (I think, mentioned in the same ISO standard you quote) might be also very useful: it will not give you a material property, but you might FE model it and see how your comparison works: in the bonded compression test the barrelling induces plenty of shear strains and might be mimiciking what happens in practice. As an additional validation, maybe a more complex test such as measuring the deflection and force exterted on the upper metallic plate on your gasket might or might not be feasible.
This is what I can say without touchin the gasket...I think very few could say that gasket-modelling is easy, so best of luck