About point 1, I would recommend you EN 13480‐3:2017, Annex Q (informative) with title Simplified pipe stress analysis.
It is not my favorite one (because is complicated in the good tradition of the European Codes), but at least is a documented way to understand how your "very senior guys" work. In fact, appart of the calculation made, they have a very good understanding of how works a particular piping system, or better said, how the system- flexible enough and well sustained- will work after "convenient" placing the restraints. Of course their experience and knowledge is hidden under the word "convenient". In brackets, when look to a formal stress analysis results, the first alarm bell would be when the results are against your intuitive expectations.

Annex Q includes a version of the "guided cantilever method". Needs practice to understand why is conservative for particular situations. Being conservative, it's a good tool for design but must be prudent when evaluate a system in terms of "pass"/ "no pass".

About placing restrictions for seismic case, a reference would be ASME B31Ea-2010 (Addenda to ASME B31E-2008). However for complex systems the "art" is to have proper restrictions for seismic (occasional) case that not restrict significantly the thermal displacements.