I'm honored that you were motivated to write your first because of the challenge.

You speak about mildew... have you tried spraying those plants with milk? It is supposed the help against mildew.

he brown spots on your plants look exactly what ws going on with my plants last year. I don't know what is cusing it, but if i were you, I would remove all affected leaves, and spray them with a neem oil and/or garlic solution.
I can't assure you that it will help.
I never found out what killed my plants last year. All I know is that is was airborn - sprread through the air - and one after another got the same brown spots and eventually died.

Once you removed all brown leaves, out them in different rooms, if you have the chance to do that. If it is the same as what has affacted my plants last year, that can keep them from infecting each other over and over again...