Ok, let's say I get the 15k, do the shadow thief quests, jam over to Spellhold, and grab Imoen. My question is this: Do I have to go with Saemon to get to the Sahuagin city? Also, if upon arriving in the Underdark, I go straight to the Kuo-toa area and the exit beyond, can I still come back after going topside and do the Drow city?

1) Yes, you need to go on Saemon's boat to do the Sahaugin city (using the portal takes you straight to the Underdark)

2) As far as I know, the Drow city is closed after you fight your way past them to the surface, whether you went there before or not (the ultimate missions wouldn't make sense if the battle had already begun).

If you go to the exit gate in the tunnels before doing all the quests for Adalon, the gate will be sealed and you won't be able to use it. The only way to get to the surface without entering the drow city is to kill her, which will open the gate. Killing her will dispel the drow illusion spell (and prevents her from casting it if she hasn't yet), so you won't be able to enter the city afterwards. In short, the only way to get into the drow city is to do her quest, and it's not available afterwards regardless of what happens in there.

You can go to all of the other Underdark areas afterwards, but not the city.

Unfortunately not, but drow weapons are pretty effective during this chapter if you lack good equipment.

It will, however, disintegrate when you go to the surface.