1: We can assume the Dwarves are the masters of porous rock and geology,as well as the mechanics of drainage. NZ has said much already,so I won't really elaborate. Isn't it possible that the Dwarves have slabs of stone to cover leaky shafts?

2: Dwarf feet are Man size- look at Hobbit's unusual feet, yet wearing dwarf-boots. And remember when Dain charged up the stairs of the East gate to decapitate Azog? Dwarves skip steps as well. Besides, those stairs are meant to goad orcs into foolishly rushing up the stairs, where Dwarves easily whack them into the abyss.

3: Balin surely is being naive- though Dwarf-records don't mention it (Durin's Bane) as being a Balrog of Morgoth, he should have understood the danger.  Old records say the 21st Hall, but Moria may well be deeper. the broken and torn Book of mazarbul might have other replicas, with similiarly innacurate information.

4:Look at Erebor. It has a few Public entrances, like the Front Gate but has a secret enrtance or two. Moria has two large gates, one for Hollin and another for Dimrill. But there are definetely other exits - many of Durin's Folk escaped Moria, but not Erebor. You could assume the Eastern side had more doors- where would the Iron Hills crew truck in all their iron ore otherwise? These are Tradesemens' Entrances- Dwarf height and width, not very luminated. And not very known, either.

5: That bridge leads to Public Entrance #2. it could be reinforced with steel and iron foundations (like Barad-Dur). Certainly the Dwarves know of this art.

CHALLENGE: Took em to their Man-Friends and built their houses with them. Dale was probably built with Dwarf-stones left over from Erebor. Dwarves don't waste stone, I think.

There is but one task remaining in Arda-we,the Dragons of Valinor will make it happen.