Yes. There is a window on the west side of the south west room in Elrath's house, which you can throw a teleporter pyramid through. It is at the northern corner; there is a window on the east wall and a couple small ones on the south, but I was not able to get a teleporter stone through them. In detail:

https://www.dropbox.com/s/qu58mi1scrym6qz/elrathbreakin.jpg?dl=0


Alternately (probably not as easy), you can get back in the basement either by hex editing (Teleporting Anywhere) or file swapping (FIX for the Missing Catacombs Portal), or replace the set of potions (Lost Items - guide and directory of inventory files).


By design there are only 2 cure potions in the game, but 3 victims. There are ways around that, though.
You can Pickpocket (level 2) a potion back from a cured person, kill one after curing (with no loss of reputation) to get them to drop the potion, or duplicate the potions before using either.

To copy a stackable item, drag a stack out of (or into) your inventory (or container) and hold down the control key before dropping. This will bring up the split stack pane; type in a negative number and hit enter (the checkmark will be greyed out). This can also be done in the trade window.