"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."

Grooves like gutters might divert drainage to one of several potable water chambers throughout the mine. The permafrost of the mountain heights might limit the amount of actual water leakage, as most precipitation would freeze.

"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."

Given their tirelessness in carving and mining, Dwarves probably do enjoy a good stairway and do not tire as easily as we do of them.

"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? "

Works for me.

"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?"

Certainly. That the narrow Bridge of Khazad-dum was considered a defense implies that the ways in and out were somewhat more accessible.

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

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

Made mortar of them for building and forging. Possibly smelted them for trace metals. And then there's that big ol' abyss as the ultimate garbage dump!

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"In that hour I looked on Aragorn and thought how great and terrible a Lord he might have become in the strength of his will, had he taken the Ring to himself. Not for naught does Mordor fear him."