I killed Marcidia and looted the tower. Now, the next part in the onine walkthrough I have seen is something like ...

"... go to the cliff above the giant flower. Interact with it. Cephal will leave at this point..."

I have been spinning my wheels running all around the Upper Reaches, Depths, Tower, etc. .... and I can't figure out how to progress to this point so I can leave. Basically? I am lost in Lostlarn Keep with no sense on how to leave.

At the end of the water part of the dungeon, you will see a vine wall. Interact with it and go interact with the big flower near by.

Okay. I am all prepped and saved.
We are in "Lostlarn Keep - The Depths."
I am in the far northeast where there are two big flowers .... and a Jabberwock ... a 26 HD dragon!!!

There aren't any choices for me to interact with either flower. I can send a picture to show where I am on the map, and in-game, if that helps.

But otherwise, I am finding myself stuck. Attached are the current placement - physically, and map view.

Yes, on your map screenshot. About 25 meters north you will see a wall... look at it. I think a magnifying glass will show up and inspect/interact with it. Then go back to that flower.

https://www.youtube.com/watch?v=Jf64Qyc9nsU