Have you taken a fecal sample to the vet?
There are different strains of cocci, and if you treated with sulmet for the entire duration of treatment to no effect, it is possible she has a resist strain of the protozoa and a different cocci med may work. The most common strain is the only one with blood int he poo and so on, but there are other ones out there which by now could be resistant to the commonly available drugs.
As for worms. It is also possible she still has them but they are now resistant too if you were just that unlucky to have created them during the last time they were wormed. How is her weight? Any respiratory issues or just listless with with no appetite?
Cocci or worms can cause listlessness and weight loss, but the use of wormers and other drugs can also damage the flora in the intestines and cause diarrhea if they get too much at one time. If treatment is to be through water, mix according to bottle and don't force them to drink it, as it may cause a dosage overload if the body isn't working right. You probably don't want to mix drugs either as that could damage the liver and other internals.
I personally don't think the primary issues she has in bacterial, but if you do treat with antibacterials, finish the whole course and try not to over use such things as they may make the same family of drugs useless in the future for even yourself as even harmless bacteria can gain resistance and pass it to all other bacteria in the environment.
As for contact with things that aren't just right, even the overhead bird or way word bug popping through their run could be a vector for disease. Did you take your other bird for a necrocopsy to find out why she died? That may have been a good key to figuring out what is going on.
Do heed warnings to not mix drugs and finish treatments before moving on.