You ask a really good question to which I don't know the answer to but can provide some observations!

First, good news for your dry eyes. You can use an evaporative type humidifier with certain precautions. I use one during the winter months and it's a life saver. Below is an excerpt from Dr. Falkinham's paper on "Reducing Exposure to Nontuberculous Mycobacteria (NTM)". He describes how you can safely use an evaporative humidifier.

"Humidifiers: A Big Problem. Humidifiers, whether small room-size or whole
house HVAC systems, transfer water to air. They can also transfer microorganisms,
especially NTM, Legionella pneumophila, and Pseudomonas aeruginosa. Transferring NTM
from water to air is the problem. Humidity helps breathing, but it exposes one to
NTM-laden aerosols. As humidifiers come in two sizes, room and house-size with
different considerations, they will be dealt with separately.
Room-Size Humidifiers. Small, room-size humidifiers can generate aerosols
with high numbers of NTM; even from reservoir water containing relatively low
numbers of NTM (500 CFU/mL). In particular, the new ultrasonic humidifiers generate
a high density aerosol mist that are rich in NTM (Hamilton and Falkinham, 2018). If
you need to humidify your air (especially during winter), only use a room evaporative
humidifier. Evaporative humidifiers boil water and the mist is blown by a fan or
simply rises. Boiling will kill any NTM or bacterial cells and thereby be relatively free
of NTM. The ultrasonic humidifiers do not heat the water, but use ultrasonic vibration
to produce aerosol droplets. If a room-size humidifier blows water through a wetted
filter (called an evaporative humidifier), don’t use it as the water wetting the filter will
have NTM that can be aerosolized.
Remember that NTM will grow in biofilms in the humidifier’s water reservoir.
Therefore, scrub the surface of the reservoir before refilling to reduce the number of
NTM and other waterborne pathogens in the biofilm. I suggest you use a bathroom
scrubbing agent. You can even disinfect the reservoir by adding Clorox® (1 cup to 10
cups water) for 30 min. Then be sure to rinse repeatedly until you can’t smell the
chlorine before use.
Household-Size Humidifiers (HVAC). In an on-going study of NTM-patients in
Philadelphia (the same hospital and area where the elderly, slender women were first
identified at risk for NTM pulmonary disease), our colleagues at the Lankenau Medical
Research Institute (led by Dr. Leah Lande) discovered that all the NTM-infected
women have whole house humidifiers that are simple fabric or plastic woven filters
with a channel above with holes for tap water to drip down through the filter. Those
systems are like the “swamp coolers” used in the desert southwest to cool homes. The
wet filter cools the air during passage. However, NTM adhere to the filter material
where they grow and are transferred the household air that is drawn through the filter.
Such house-size humidifiers are difficult, if not impossible to disinfect. Theoretically, if
the incoming water to the filter was sterilized by 0.2 micrometer filtration or ultraviolet
(UV) irradiation, the filter was disinfected regularly like a showerhead, the duct work
leading in and out of the humidifier throughout the whole house was free of NTN, and
the incoming air was free from outside dust, the humidified air would be relatively
free from NTM.
Room- Versus House-Size Humidifiers. I prefer the room-size as they are
portable and easier to clean (biofilm-removal) and disinfect (Clorox®). Successful
employment of a whole house humidification system requires disinfection of the
existing ductwork, a daunting task."

Regarding, your question, "If MAC is all around us, and I have it inside me anyway, is there much benefit to me taking all these actions to get rid of it?" So far, science/medicine does not know how much exposure (load) it takes for us to become infected with NTM. Is it one single exposure or multiple exposures over time? Does continued exposure increase NTM infections if we already have them?

We know where it resides in the environment and which activities expose us to large loads or concentrated NTM. We also know that some pre-existing condition makes us susceptible to NTM, in our case bronchiectasis. From there we make a hypothesis: avoiding NTM in our environment will decrease our load exposure. That combined with airway clearance and possibly drug cocktails will help to eliminate or tap down NTM infections and slow bronchiectasis progression. The trouble is that there is not an efficient way to test this hypothesis, which is why this disease and NTM frustrates us! And you ask good questions like you did! I hope that the Honda lab is working on answers to these questions. Until then, we have fairly manageable modifications to our daily lives to limit exposure if we decide to take a conservative approach. My 2 cents.