A large portion of the world already wears masks regularly in the form of niqabs. I'm sure there's some research out there on niqab-wearing that could help support or falsify some of your hypotheses. It may be worth looking into if you are interested in pursuing this question further. For future posts on this I would also be interested in what you think would be most likely to lead to most of the world wearing masks. I think that would shape a lot of how masks are used and the effects and social connotations that they have.

1. I have always enjoyed the Omelas story and I think this is a useful thought. This feels like an extension of some of the ideas in The emotional dog and its rational tail to handle cognitive dissonance. When we are faced with large societal issues (whether utilitarian or not) that we find objectionable, we are faced with two choices. We can either change our actions to put ourselves in opposition to the practice, or we can figure out a way to justify it morally. The latter can often be the path of least resistance and I would put 74% odds that it is the one that leads to higher subjective well-being. I don't know of a good way to solve this but it seems like a very large flaw for democracy.
2. That being said, there are absolutely things that people talk and think about as "necessary evils" where they find it objectionable but it satisfies some greater utilitarian goal. The way people often talk about the nuclear bombing of Hiroshima and Nagasaki comes to mind as an example. I have a few similar examples I can think of but I am trying to avoid throwing in anything politically incendiary. I would be very curious what about a person/issue can create this "necessary evil" view, vs. trying to find some sort of justification of why a thing is actually good. My knee-jerk opinion is that it is probably very helpful to view highly utilitarian things through the "necessary evil" lens as opposed to seeing them as completely good or bad. It's possible that it fosters a "slow change" paradigm (let's figure out how we can get the benefits of Emolas while minimizing torture to this child). This may be helpful in being experimental and purposeful with policy to get the utilitarian benefits of a thing without the ethical downsides, which I think could be more helpful than something more disruptive. This is my first time thinking about any of this though.