Harvey said:
I wasn't aware that this was a competition.
Yes, a Google search turns up 14,200 hits for みんなさん. This is compared to slightly under FIVE MILLION hits for みなさん and nearly EIGHT million for 皆さん (note that 「みなさん」 is the only standard reading for this). That's a far cry from saying, as you suggested, that the two are equally common or completely interchangeable.
If you search the internet, I'm sure you could also come up with a bunch of examples of English speakers who confuse "it's"/"its", "there"/"their"/"they're", "too"/"to", "lose"/"loose", and so forth. Some are probably typos, others are people who don't know (and don't care) about using their own language properly. Does that mean that these things "don't really matter" when you write English?
Besides, seeing みんなさん on webpages doesn't mean that a significant number of people use it in speech. Many English speakers misspell "lose" as "loose", but I would imagine that not a single one actually pronounces it like the adjective "loose".
I have a feeling that if you wrote みんなさん on your blog or personal homepage or whatever, you could get away with it -- most people would probably think it was a typo. This doesn't change the fact that if you were to pronounce in that way when you speak, the overwhelming majority of native speakers would find it very strange.
You already have a few people (including a native speaker) telling you this. It's your call what to do with that advice, but you may want to at least consider the possibility that you might be mistaken.