I would require the client document what they consider 'remote contingencies' and any other exception they take to API 520 recommended practice. If they do not define the scenario as an exception, then consider it no exception. I think the client could make a case for 'remote contingency' for scenarios of full tube rupture and both control valve and bypass wide open. I would not be comfortable putting continually connected NNF startup lines or maintenance nitrogen lines in the same category since these WILL be used according to plan.
Some engineering companies would NOT allow their engineers to design to a lower standard no matter what the client wants. If you are part of a large, reputable engineering company, then your chief engineer and perhaps the legal team should be involved as it does not seem your authority to commit your company to possible legal hazards. If your company does not care about such things, then my suggestion is to document your calcs clearly showing the basis for allowable overpressure. Record calculations for both standard and exception to defend the later charge of ignorance and incompetence.
It is of no consequence to anyone if the relaxed guidelines do not affect the pressure relief design. Where the relaxed guidelines result in 'less protection' then you need someone else (preferably the client) to accept the responsibility. Your company and the client should agree to the guidelines in writing with explicit acknowledgement that they violate recommended practice.
Will you be able to defend your engineering actions before a jury if the occasion arises? Generally accepted engineering standards exist for a reason. Do not ever perform any engineering function that is reasonably likely to harm someone's health or safety. You have a responsibility to the public that overrides your responsibility to your employer or client.