Actually, LLNL (the site of El Capitan) has a process for requesting Dedicated Application Time (a DAT) where you use up to a whole machine, usually over a weekend. They occur fairly regularly. Mostly it's lots of individual users and jobs, like you said though.
Steels can undergo a transition to becoming brittle when they get cold (called a “ductile-to-brittle” transition). It’s important to know what the properties would be like in this regime and -70C is enough to get there (even 0C can be enough, depending on the alloy).
The reason this person may have thought the -70C test eqs stupid is because a sub will never be working in conditions much colder than the freezing temperature of water (which is not strongly pressure dependent, btw), since the water would want to freeze - not good for the boat.
It's not "initial conditions" as in time=0. Thermodynamic state variables are path independent, so "initial conditions" in this context means the conditions of the reference point in the process from the point you start measuring.