Hi again, thanks for the information!
There are many things that come to mind for a cat that age...
Certainly respiratory infections can cause some of the symptoms you are seeing, including pneumonia.... and feline leukemia can be an underlying cause. Feline leukemia is a virus that causes the immune system to not function properly, so cats get very sick from infections that cats with healthy immune systems would be able to handle. A simple blood test can determine if that is part of the problem. If so, the chances of recovery are much poorer. FIV is also a viral infection that can affect cats in a similar way. It can be easily tested for as well.
Other things that come to mind are heart disease, hyperthyroidism, diabetes, and kidney disease, all of which can be checked for with a blood panel at a vet's office. Man of these conditions can be managed with medications and diet changes, especially if feline leukemia or FIV are fuled out. Also, unfortunately, in older cats we have to consider cancer as a cause of illness, too. Sometimes we can detect changes on a physical exam that clue us in as to what might be going on, so that's where I would start. I'm afraid that there's not a lot you can do at home with the medications you have that will help, in fact, cats are very sensitive to human over the counter medications and they should never be given unless instructed by your vet as they can cause more damage than good.
I hope this is helpful in helping you know what might be some of the causes of what you are seeing. I encourage you to seek out a vet for an exam and a test for FeLV/FIV, at least to start Call around and get some prices for what that would cost. Also, some humane societies and shelters offer low cost vet services or may be able to recommend a vet to you.
Good luck and please let me know if you have any additional questions.