1.  How do the Dwarves keep water from coming down their light-shafts?   For that matter, how do they arrainge drainage throughout the whole of Moria?  It’s a major concern in human mines of any serious depth.

That might be one reason why there aren't a lot of light-shafts and the ones they did have were strategically placed.

2.  How high ARE the risers of Dwarvish steps?  I had assumed 2/3 of a normal human step, since Dwarves are about 2/3 of normal human size.  But Gandalf sits in presumed comfort of the FIRST step of the last staircase.  And some readers insist that the Dwarves LIKE climbing stairs.

Maybe Gandalf has a skinny butt.  I would agree the stairs are 2/3 of what we use.  If I was building stairs, I would size them for myself - I wouldn't be thinking "Hey, let's make them extra big because I want to climb stairs like that for the rest of my life"

3.  By the way, since I havn’t said it already, I need to point out that Balin obviously chose the twenty-first hall for a residence not only because it had light, but because it is the extreme corner of Moria, and therefore the most defensible.  Comment?

I would agree

4.  Is it really feasable to have such a large city with only two rather constricted exits?  Should we not presume many dwarf-doors, secret and now forgotten?

I don't think there were all that many dwarves at any one time, but of course there were secret doors.

5.  What could that bridge be made of ?  A single slender arch of fifty feet that has stood unmaintained for over a thousand years?

Good engineering

CHALLENGE

As the Dwarves hollowed out these truly vast works extending for miles under three mountains, what did they do with the tailings?

Some of the caves already existed, and the dwarves merely joined them with passages they built.  I would guess they threw  some of the tailings down some of the pits we keep stumbling on.  Maybe that's what awakened the Balrog.

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Then with a crash came a great ringing shout: ra-hoom-rah!  The trees quivered and bent as if a gust had struck them.  There was another pause, and then a marching music began like solemn drums, and above the rolling beats and booms  there welled voices singing high and strong.

We come, we come with roll of drum: ta-runda runda runda rom!