0: I assume they don't pronounce it as mas-TO-don, since that would be a rather bizzare stress pattern, so the "o" generally gets weakened to a schwa as is typical for unstressed vowels.
See, I think this is definitely why I'm confused, because I can't for the life of me think of anything that fits this pattern. Are you talking about japanese onamonapias, like kabedon? that's the only word I can think of with an ending that might possibly be analyzed like that. a quick search of wiktionary for words ending in "adon" doesn't turn up anything obvious that might fit that pattern either. Could you give some examples?
I pronounce it rougly /mæstədan/ and I imagine many other people do as well. That sounds is closer to "a" than "o".
So is "helicopter", but you don't typically break it apart as "helico" and "pter" in order to pronounce or spell it.
Second, language is fluid... if you think it shouldn't then good luck policing that.
Third, I feel your pain. I groan everytime I hear "which begs the question".
I see the same thing all the time with Etherium/Ethereum as well for example.
I had the same problem when NGINX first came out. It wasn't until maybe a year later after starting to use it that I actually talked with another person AFK about NGINX, and they said "engine-x" which I had never heard of.