Re: Impulse Deflection Crystal...?
"Designed to operate separately from the rest of the ship, the saucer therefore contains all elements necessary for independent operation." "Minimal crew quarters are located in this (secondary) hull, used by duty engineers and by the star-drive crew when the saucer section has detached and is operating separately."
Making of Star Trek,
Considering they already considered for TOS that the main and the engineering hull ("star drive section") could occasionally operate separately
"Did they? All dialogue references are to discarding
one component or another, rather than to continuing to operate it after a separation. It seems that a discarding sequence leaves just the saucer section surviving to tell the tale - and the propulsive capabilities of that section are never made clear, save for its supposed ability to break from orbit and escape the episode's calamity that is dragging the rest of the ship to a fiery crash."
Part II, Chapter 2 (The U.S.S. Enterprise), pages 171 and 191.
Yes, they did consider hull separation for other occasions than just emergencies.
"The first duty of every Starfleet officer is to the truth" Jean-Luc Picard
"We can't solve problems by using the same kind of thinking we used when we created them." Albert Einstein