[HerenIstarion]

It is not certain that dwarves ever planned leaving Moria at all, and that is the reason of having such a narrow bridge and opportunity of bottle-necks and whatever. Moria was first place of awakening of dwarvish forefather, of Durin himself indeed, so it was impossible for any given longbeard dwarf to even imagine that his race would be forced out of Khazad-dûm. So there were no secret doors on the west side and so on as it was in Erebor - a kingdom smaller an newer by far than Moria, where all fortification were outword oriented.After all, nobody expected a balrog as a home danger. So there was no planning of leaving Moria in large numbers in haste (save marching to war, but that would be well ordered and would not cause bottle-neck), as dwarves ever retreated inwards, not outwards.