About your first point, I guess that your model includes also an above ground section (pig facility, for example) and for that part you should perform a seismic evaluation as usual.
For underground section of pipelines, seismic hazard is either ground deformations (soil failure, liquefaction induced buoyancy effects, landslides, faulting- relative movement between two portions of the earth crust, etc) or wave propagation (P, S, Rayleigh type, etc).
The principal failure modes of pipeline are rupture due to axial tension, local buckling due to axial compression, beam buckling and bending failure. The problems are more complicated when local corrosion appeared in service.
The point is such analysis should be performed with proper tools; IMO it is better to say frankly you have no tools instead to say "I made an analysis, there is no problem".
Again IMO, such seismic analysis for pipelines remained more linked to academics, while in real life focus is to mitigation strategies.
Best regards.
PS. On this subject, you can find huge amount information on Internet and study it.
You can start with
http://www.americanlifelinesalliance.com/pdf/SeismicGuidelines_WaterPipelines_P1.pdf
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