Would we reach a point in a normal dynamic situation (timescale minutes) where differential pressure exceeds the rating of the negative pressure relief valve and attains -1.45 PSI difference, for several minutes?
The negative pressure relief valve was designed for
rapid descents, but Boeing words the manual carefully:
This can prevent structure damage during a rapid descent.
I assume they are designed for full gear/spoilers extended max airspeed rapid descents.
The relief valve is simply a flapper door with a spring on it (low tech). Like all springs, they will have an operating range. They are not light switches with instantaneous on/off actions. They "meter" the flow (according to the magnitude of differential pressure)
Door (rubber seals) don't work well with negative pressure, so they may let air in (small amounts)
Door sill drains do not normally vent in the air. Normally, any water present in flight will be kept in "bladders" and vented "on the ground" (i.e. at 0.5psid). The valves actually close at 2~5psid. Again, we are talking about small leakages.
Drain masts,
as far as I know, do not have valves. I've seen water, cola, tea, etc pouring from these (onto unsuspecting people on the ramp) at a relatively high rate, but again, we are talking about relatively small leaks (compared to outflow valves and negative pressure relief doors. There are anti-siphon valves between the sinks and toilets to prevent toilet water backflowing into the sinks, but I'm not sure how they would react with negative pressure differentials (if at all).
I am not sure what you mean in your first sentence, as it is not a normal depressurization scenario where dP=0 from the trigger event.
Sorry, I was trying to understand what your biggest concern was ... achieving a breathable atmosphere (pax concern) or negative pressure differential (fuselage integrity).