emoji's are part of text communication in the same way facial expressions and body language are part of oral communication. Yes, sometimes they'll be misinterpreted, but the same is true of text alone anyway (were they being sarcastic? Are they angry?)
That's exactly what emoticons were meant to substitute: non-verbal communication. The problem is that some use them as hieroglyphs.
Well, that's those people's problem. They won't be able to communicate with anybody outside of their group, so they must eventually give the style away or learn to tone it down when talking to strangers.
Not differently from any other kind of group idiom people created at any time in history.
I think you're jumping to conclusions too quickly. It is true, emojis can be an important part of communication but until the same graphics start being used on all platforms, devices and services, emojis are a quite bad medium for communication. My phone, my browser, whatsapp, slack, I think twitter too and so on use their own graphics™ which have subtle differences.