"1.  Why was there not a serious force waiting for the Company on the west side of the bridge?"

Many reasons: a) The bulk of the orcs would be deeper inside Moria. b) the fire trap failed. c) the Balrog fell. Basically, the orcs never imagined the Company even reach the Bridge, much less actually defeat the Balrog and cross it.

"2.  How could this guard force not be fully alerted and ready for anything?"

See above. Also consider that the Balrog's guiding will was now lost to the Moria orcs.

"3.  Why in the WORLD didn’t the forces of evil concentrate on holding the exit, instead of wandering around in a maze and getting cut off by their own fire?"

The forces of evil aren't very capable of strategy without a guiding will capable of such strategy. Was the Balrog that will? We don't know. Also, the orcs may not have effectively mapped out the layout of Khazad-dum. Orcs are more tunnelers than architects like the Dwarves.

"4.  In fact, since the orcs in the chamber of Mazarbul already knew about the fire, how is it that they were cut off at all?  There were all kinds of connecting passages."

They probably misjudged where the Fellowship's road would exit or were unaware of all its potential exits.

"5.  Is it possible that there aren’t really very many orcs in Moria, and that they just don’t know the place well?  And/or that the orc in black armor that speared Frodo was the only high-initiative individual in the place? "

Uh...yeah. Without a will to keep them "on-task," and out of fear of the Balrog, the Moria orcs may have depleted themselves engaging in their tribal warfare and cannibalism, keeping to very specific areas of the huge mine.

"CHALLENGE
What do you suppose Sauron heard about this entire encounter (Balrog and all), and what did he think about it?"

He certainly learned that the mysterious Company had crossed over or under the mountains to enter Lorien. During the Company's stay there, he may have learned more specifics of the Moria events. Sauron probably regretted the loss of the Balrog as a potential lieutenant but cheered the loss of it as a potential rival and cheered the loss of the dratted wizard as well.

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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."