All these ambiguities disappear if you choose a reference frame. From our reference frame, the explosion hasn’t happened. From the stars’, it might have. The issue is popular imagination, and thus common language, deals with reference frames sloppily.
I deals with it fantastically, unless otherwise stated it's implied we are talking in the local reference frame. That convention gives us nearly infinite information density because we don't have to state it.
Regarding the CMB, I have no problem stating that the Big Bang is happening now in a distant part of the universe. That is an interpretation that is easy for humans to understand, and it's not really wrong since we are now receiving the photons. As a very poor Newtonian analogy one could imagine it to be similar to how a flame front in a combustion chamber propagates.
As for near-real-time two-way communication, I don't think that humans will ever be developed enough to accept the non-agreeing simultaneity. We will continue to say that Mark Watney said something 15 minutes ago when we hear his voice. For practical purposes, it is the best way to deal with the situation, even if it is technically wrong. There is precedent with knowingly using terms incorrectly, just off the top of my head we (metric-using people) typically answer with our mass, not our weight, when asked what we weigh. If I were to tell someone that I weigh 700 Newtons they would have no idea what I'm talking about.
This may seem silly, but this is how reality works and as humans adjust to opperating in multiple spaciotemporal frames of reference, our language will have to adapt
This is only true if you spend too long navel gazing.
Relativity may have made a strict, absolute, universal "now" and "simultaneous" obsolete or non-existent, but language can't and won't adapt to the absurdity of claiming they don't exist. No one was making grand claims about absolute certainty of simultaneity before relativity, saying it doesn't exist has really only minor effects.
Language is contextual and with context it is indeed silly to claim "now" doesn't exist. Language can understand frames of reference and acknowledge disagreements without throwing out everything.
Unless photons and black holes start having conversations.
Not at all. "Now" can only have meaning withing a spaciotemporal frame of reference. We are used to operating within a single such context and our language assumes that.
Your point 2) explicitly brings up people from two different contexts discussing what "now" means.
While language that assumes a single spaciotemporal reference frame will continue to be used and to be useful, that doesn't make it useful for discussing systems with multiple frames of spaciotemporal reference.
1) The cosmic microwave background is, in short, the light still traveling from an early stage in the big bang when the universe turned transparent (380,000 years after the big bang, when the universe became cool enough to form hydrogen atoms)
You can see the big bang happening. So when would you say this phase transition to transparency happened? Here you would say it happened 13.5 billion years ago. At the source of the CMB photons you can see, you would say that is happening now.
With that language construct you have an event which happened across the universe simultaneously happening both 13.5 billion years ago AND right now, depending on location. Which, I think, is absurd.
2) Talking to an astronaut on Mars. Mars is between 3 and 22 light-minutes away. If you want to say hello to your Martian friend, it will take between 6 and 45 minutes for your radio to play the sound of your friend saying hello in response.
What is happening while you are waiting for the response? (how do you discuss the time, what "now" means) To your friend your message has "not happened yet" while to you it already happened.
It is silly to have a definition of "to be" that must ignore the practicalities of simultaneous events which involve non-trivial light transit time. It is very easy to talk about a "now" where both Earth and Mars have the same timeline dispensing with the light cone and talking about light travel taking time. It is not so easy the other way around.