Over at this post http://rolesrules.blogspot.com/2010/09/is-underground-railroad.html at Roles, Rules and Rolls there's a good piece on railroading players in the underground. go read the link, come back here afterward, I'll be waiting.
Okay let's look at each situation and how to solve the bad DM dilemma so one isn't a crappy DM.
"Oh, don't go down those stairs. I haven't drawn the third level yet."
The DM is in a pinch there are stairs but nothing beyond them.
Solution #1. Just don't tell the players about the stairs. Sure they are on the Dm's map but the players don't know that. Bring it up agan later if the players ever go that way. Pop it on a map in a treasure horde sometime later after the third level is ready. Just don't bring it up yet.
Solution #2. The stairway is a horrible deathtrap. If the party climbs on down the stairs the stairs disappear and seriously screw up the party leaving them in a really bad pit trap. Cruel, sure it is but it'll more then make up for having nothing else beyond the stairs. Later on(when mapping is done) it'll be disabled, let them think the PCs had something to do with it and hey look a whole new dungeon level.
Solution #3. the stairs go somewhere else for now. Just have the stairs lead to somewhere else in the dungeon until you finish things up on the next level. The stairs are "magic".
"You have to pull the lever, or there's no more adventure. Trust me."
Solution #1. don't do this as a DM, don't "break the wall" and tell the players what to do to move things along. Want to do something else tonight, have the party go shopping after they leave the adventure site where there is nothing left to do.
Solution #2. Be a dick - have a hireling/henchman/follower/familiar/crazy old hermit pull the lever. It's still horrible railroading you didn't break the wall and yuo'll certainly make the NPc a littel more memorable to the players. Think of it as accidentally knocking that bucket over in the mines of moria.
Solution #3. Act like you don't want the players pulling the lever, they most likely will pull the lever.
"You can't turn back to the town. You haven't cleared enough rooms this session yet. OK, if you insist - A Mysterious Force Blocks You."
Solution #1. There is no such thing as "clearing enough rooms" this is not a reason for DM intervention. Let the players do what they want.
Solution #2. Let the players turn back to town and then have them mocked for being wimps and turning back. Just have stats for a bunch of buildings and NPCs yuo are willing to watch fireballed handy.
Solution #3. I don't have one, let the players do what they want.
"Your Hold Person mysteriously fails - that 1 I rolled is actually a 16. The sorcerer drinks a potion, cackles 'I'll see you on Level 6', turns gaseous and disappears down the grate."
Solution #1. Oh well this NPC is a goner, come up with another one for later.
Solution #2. This is not the NPC you are looking for. A really second rate trick but you can get away with it once or twice in the life of a campaign. The NPC they think they are fighting just isn't the creep they are looking for, that guys down on level 6.
Solution #3. The spell works but...as the spell takes effect a contigency spell kicks in whisking the NPC away (do this very rarely if ever). NPCs can prepare for later too.
Solution #4. The spell works but...as the spell takes effect the sorceror stumbles accidently triggering a trap door that drops him down onto the 6th level. If the players climb down quickly th eNPC is a goner get a new one, if they dilly-dally they climb down and the NPC is gone to be found later.
Solution #5. Ming's Ring. One of the hireling/henchman/follower/familiar/crazy old hermits with the party finds a ring/clasp/earring/lucky c.p./pair of socks on the sorcerer the PCs disregarded that the hireling/henchman/follower/familiar/crazy old hermit take home after the sorcerer is defeated. Unfortunately they are possessed by the spirit of the NPC rather quickly and if no one notices eventually are a threat for the party down on level 6.
(reading mind-molesting boxed text) "Seeing the skeletal lovers' embrace, you cannot help but sigh, shed a tear, blow your nose, and think of lost loves of your past. Then the lovers stir and turn their bony skulls towards you, and you scream in bowel-loosening terror!"
Solution #1. Ignore the boxed text.
Solution #2. Just the facts jack A. Paraphrase the box text with what you the DM feel to be important, never tell the PCs how they feel.
Solution #3. Just the facts jack B. Paraphrase the box text with what you the DM feel to be important, never tell the PCs how they feel but do tell the PCs how a hireling/henchman/follower/familiar/crazy old hermit in the party feels about the situation on hand.
Just a few pointers that came to me after reading the OP that show other ways to deal with the railroading problems that might come up. Let the players help you tell the story, never tell the story to them or try to tell the story through the PCs as if they are puppets.