Thank you very much, that helps a great deal. I am not disputing that Abby has some pain in her back from spinal disk disease, as the rear limb issues may be consistent with that...but I do not think that the back problem is related to the staring, panting, confusion, etc. The fact that Abby seems to be worse at night is also telling.
The confusion your dog is experiencing is most likely the result of canine cognitive dysfunction, also known as senility or dementia. At 8 years old it may on its surface seem strange for a dog to have this, but English Mastiffs are wonderful dogs, but unfortunately they do not age gracefully. At 8 years old, she is already a very old girl. Advanced age and genetic predisposition just like senility in people, is what contributes to progression of disease. What is hallmark to me, is that the signs are worse at night...a very common finding in cases of canine cognitive dysfunction.
My advice to you would be to continue managing the suspected back disease, but print out this consultation and discuss concurrent management of cognitive dysfunction. Management of the disease is a two fold approach that combines medication and prescription diet.
The medication these cases commonly respond quite favorably to is selegiline, administered once daily by mouth. It not only can be very effective in keeping the senility at bay, but it is quite safe and has virtually no side effects. it should not adversely react with a any of the medications she is on.
Next, I would put her on the prescription anti-senility prescription diet called Hills B/D. It is specially formulated to reduce free radical formation that can exacerbate senility, while also feeding the body with very brain friendly nutrients.
My experience is that like in people, this is a battle that we are eventually poised to lose, as this condition gets worse over time, not better. However, since treatment is simple, often effective, and not terribly expensive, I see no down side to trying to give her (and you) some relief and potentially provide a 6-12 months remission.