My opinion is to run model 1 and ignore all seismic displacements - do accelerations alone.

Models 2 through X will be sections of that model where you have relative displacements that are a problem, and chances are you don't actually need supports in that model, you just have pipe going from point A to B (support 1 and support 2) that are just displacements. You can assume displacements will be thermal plus relative support displacement (again, not total displacement).

Support 1 moving 100 mm and support 2 moving 50 mm is the same as saying support 1 moves 50 mm and support 2 moving 0 mm.