Based on your symptoms and my experiences, let me comment on your medication regimen:
Mucinex DM contains a cough suppressant and should not be used long term. The right Mucinex to use long-term is Mucinex Extended Release which is guaifenesin with no other additives.

Clear Lungs contains about 15 plants and herbs. I know we always assume these are harmless, but I personally cannot use it because there are ingredients in there to which I am allergic. Seeing that you already use Loratadine makes me wonder I wonder if you also may be allergic & discontinuing Clear Lungs might help?

The Neti Pot is repeatedly putting many ounces of saline water into your already overactive sinuses. When I do this, I have continuous drainage in my throat, so I only use it when I have a cold or sinus infection, and then just for a few days. Maybe it is not as helpful as you would think?

As for precautions - a mask will not prevent MAC or keep it from getting worse, but in close contact situations, a mask can prevent you from catching other respiratory illnesses. I don't believe wearing a mask in the shower would offer any protection, nor do I go to the length of boiling my water. In my (inexpert) opinion, you are getting a lot higher dose of bacteria of all sorts in your lungs from the mist above a swimming pool while doing aerobics than from showering or drinking water. Only you can decide which precautions are right for you.
Sue