Thanks for the additional replies. I just needed a bit more reassurance that I wasn't missing any major pitfalls in my plan.
I'm going to adjust things as follows:
The Kobolds will have a few javelins to throw. This will give them a bit of extra oomph and also provide the party with some extra missile weapons to take on the Mimic.
To respond to a point brought up by Meleck, if the Mimic is killed and falls into the chasm, the party should have no problem getting across the bridge. The rogue could jump that 10 feet in his sleep and if necessary, the Sorcerer can just cast Spider Climb. As a bonus, they aren't encumbered by armor!

One last note about the Mimic: I stole this idea from somebody here at the boards but I can't recall who. But thanks to whoever it was.
If they are really beat down by the time they get to the Myconids, I might have them toss in a few extra healing potions for the party so that the cleric can feel a bit more latitude to do some buffing or use spells to repair the weapons and armor they have scavenged along the way. Also note that while not exactly a stellar combatant, Quick Spore (the Myconid worker who the party will be escorting off the island) can make two +3 slam attacks for 1d6+1 so he can at least contribute a bit to the battles after he joins the group. I'm also having the king Myconid give him a couple of potions (Bulls Strength, Cure Moderate Wounds and Resist Elements) that will make him a more valuable member of the party.
After thinking about the Magmin/Ash Rat thing a bit, there is no reason that they shouldn't do just fine at that encounter. If they protect themselves from the heat (via the Sorcerers spells, the Clerics spells or the potions from the Myconids) this encounter might just be a bit of a pushover. If they don't protect themselves, they are just being dumb (the'll have the potions and the Sorcerer can cast Endure Elements any time he wants) and deserve whatever they get.
I'm going to monitor the clock as we go and if they haven't made it to the Grimlocks by 10:00, I think I'll drop that encounter out. I'll still keep the steam filled room and let them worry a bit, but no nasties will attack them. That will give me plenty of time for the climactic battle with the Pirates.
One looming question will be how they get to the sea cave to try to board the boat. The can enter through the minehead and work their way through the caves encountering some Hobgoblins and the two Ogres who were their principal jailers (which will probably be some fun revenge). Alternatively, if they are feeling sneaky, they could enter through the sea cave itself using Spider Climb (which the Sorcerer has) or maybe Water Walking by the Cleric. That way will be easier but it will use up some of their spells (then again, so will fighting their way through the caves).
Once there, I've put the Pirates (6 of them) in three distinct areas of the caves so it will take a bit for them to come to each other's aid. They are all level 4 characters and decently equipped (although not nearly as well as a PC would be) so this will be a tough battle. But the party doesn't have to kill them all, just hold them off long enough to steal a boat and paddle away.
Hopefully the timing will work out so that lava comes pouring into the sea cave just as they flee out onto the open sea.
Thanks for the help on this. If there are any other suggestions, feel free to post them. I'll check in the morning before I make my final preperations for the session.