There's no other way for you to get in to the tower, I think, unless you have a save point before then or want to cheat.
Here's what would have happened if you entered the tower (in case you're curious):
Minor Spoilers
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You would have entered, fought the lich, gotton some standard lich XP, and some goodies (the nicest of which is the crossbow of affliction +4). While going through a chest you would be transported to a place where you meet Jarlaxle, explains that he used you to kill the lich for him but then says that you're allowed to enter the house of Jae'llat where you'll encounter a good fight and some other trinkets. Personally, unless you were going through the game with the explicit purpose to play every quest possible, I wouldn't worry about missing this one.
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