Heyas Aeryn,
try this quest structure changing command:
1) open console (^-key/~-key) and enter "twoworldscheats" (without "")
2) then enter in console "quest.tq 2366" (without"")
This should move you in the quest structure to the point where you shall bring an item to Basel (this item is usually found in the sewers during a part you probably missed; or you even have the item, but missed a later step requiring conversation with now dead Yerske).
WRONG, see edit:
As the quest actually doesn't check for the item (and you skip any presteps requriing talking with Yerske) you should be able to proceed normally from here on.
Beware though as you might later on run into trouble again due to Yerske missing (might be though he is simply respawned by the quest system in any way, haven't checked). This is some kind of "quest brain surgery", so expect the unexpected

. Good luck!
Edit: More complex than I thought:
Please also enter in console:
3) "quest.tq 240"
4) "qitem_237" (and pick this one up; you might have it double now)
If this should not help, try an additional:
5) "quest.tq 241"
This last one should help in any case and move you to the next Basel quest (you miss your rewards though).
In any case go to Basel to continue.
Greetz
youngneil1
P.S: Alternatively you could also try to ressurect Yerske with:
1) open console (^-key/~-key) and enter "twoworldscheats" (without "")
2) then enter in console "NPC_209" (without"")