It would be nice if killing Kherab let you control teleportation there.

I think Grond's solution is fair enough.

The level restriction solution sounds like a good one. I would have thought, though, that instead of removing teleport control in the first place it would have made more sense to make Darkforge a level that can't be teleported around at all -- like in the elemental temples, cat lord level and the basement of the TotHK. After all, it seems that the level was designed for the PC to have to fight through all the steel golems, and teleporting past them into the treasure rooms (a.k.a. the Darkforge Raid) was regarded as something of a 'sneaky tactic'. The way things currently works still means that a PC with teleportitis that has no intentions of doing the 'raid' can get shot off to the exit corridor and surrounded by golems, which is hardly fair even if the PC is equipped with tactics for taking out the golems one by one.

Alternative solution: Teleport control works properly once the PC deals with the eternal guardian.

Alternate 3:
The more sources of teleport control the player has, the better chance of control.

1? 1/2
2? 3/4
3? 7/8

Heck, maybe TC should not be perfect everywhere. Just that some levels it's better than others. (which will kill you dead in shops if you're unlucky.)

This sounds good because you'd at least have a chance to do an early raid.

Lift the prohibition on teleport if the PC can grab the crown of science... say, maybe that evil artifact was causing teleports to misfire there.

This. If PC is at the stage where he/she can use the shortcut from D:32 then teleportation should be allowed in DF.

I prefer this idea to all others. Kherab is a very tough opponent and most of the time players don't have much motivation to kill him, this would provide an incentive.

And let me add: for chaotics, completing his quests should also make him lift the teleport restriction.

I have lifted the teleport restriction once Kherab is killed. Lifting the restrictions once you fulfill his quests doesn't sound site logical to me - why would he want to increase the chance of raids on his storage rooms just because you have done a couple of things for him? If he wanted you to gain more stuff he'd give it to you directly...

Yes, that's logical. I think lifting the restriction when Kherab dies is a good solution.