Well - strictly speaking - you don't need to break the alliance with the Illithid.
If you do the other areas, then you have completed enough.
The ways to weaken the Valsharess are to make it so she dosen't have undead, beholders and/or Illithids in her army.
And then you can strenghent the rebellion by getting the Golems to help you and a Deva. (Although - I don't think the Deva counts as the others, because I found no way of telling this to the Seer).
I'll not spoil you to the locations and similar of these items, but my point is - based on my own experiences - is that you don't need to remove all the allies to the Valsharess. The first time I played through, I went back to the Seer and told that I had broken off the Valsharess' alliance with the Beholders and the Undead. And I had brought allies in form of Golems (and the Deva also, but couldn't tell her that).
And this was enough to progress. Of couse, it means you fight a slightly stronger army, but it shouldn't really pose much threat to you at this level.
So basically - in conclusion of this long-ish post. Don't sweat not completing the Illithids thing. Kill as many as you can and give the mirror to the Seer instead. It is a much "nicer" thing (and gives nicer ending) to do in my oppinion also
Hope this helps.
Of couse if you want to complete the Illithid - I don't know of any way to do that withouth reloading to earlier save.
Linear Mode
