The oily taste & rust in the tanks are the biggest risks, but even the course material you teach your students cautions against diving such, doesn't it? And if breathing oily air doesn't kill you, then you have to wonder how much Carbon Monoxide is getting in your tanks too? I doubt that personal CO tank analyzers are common there, but I won't dive any tank without testing for that - along with the old smell & taste testing.
Additionally agreeing with previously stated opinions here, your ear problems sound like a combination of injury & infection. Since you are an Instructor, I suppose you know the various ways to equalize - but it's got to be done well. You can prevent ear canal infections with a very cheap mix of white vinegar & alcohol, soaking in each ear for 3 minutes at the end of a dive day. Or after problems set in, with your symptoms - I hope you continue the oral antibiotics for systemic treatment of the infection plus antibiotic ear drops containing hydrocortisone for the pain. I did not get ear infections from swimming for decades, but after I started Scuba - I incurred some bad ones, so I don't go in water now without having the prevention drops and treatment drops both available. I hate sitting in a doc's office on the verge of tears waiting my turn for help.
And if you are going to take Sudafed to help open your sinuses, make sure it has pseudoephedrine in contents - the chemical that originally gave the med its name. It's more difficult to obtain that the OTC meds that do not contain it, but may have the same Sudafed brand label - but it's still the best.
A word of caution about decongestant nose sprays too: Read the warning labels closely if you use one. I was chatting yesterday with my home dive bud about his current head cold & treatment and his employee who sprays every day for allergies and they gave me the names of their sprays - which I looked up on the net. Both brands had serious dependency warnings, which neither of them knew about.
good luck...!!