Flubenvet (Flubendazole) is mainly found in Europe/UK, etc.  I don't think I've ever seen it sold in the U.S.
If you wish to deworm, then I'd use Valbazen (Albendazole) or Fenbendazole (Safeguard).  Dosing is below.
I do agree, if you lose this one, having a necropsy through your state lab will give you the most information.
With poop like that, she may have some type of reproductive disorder, it can be hard to know.  Unfortunately, reproductive disorders are common in laying hens and sometimes the best we can do is manage symptoms when they arise and keep the bird hydrated and eating to see if they can rally for a while longer.
I'd still give the Extra Calcium for a total of 5-7 days, this often can help if they are having a hard time expelling material.  Check to make sure her crop is emptying and give her an over all look for any externa parasites (lice/mites) just to stay on top of thing. 
Safeguard dose is 0.23ml per pound of weight given orally once a day for 5 days in a row.
---OR---
Valbazen dose is 0.08ml per pound of weight given orally once, then repeated in 10 days
Here's how to give oral medications:
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/