Does the inevitable end to the universe mean that all human action is irrelevant, except for one that might perhaps stop the end of the universe?
More or less, but not because of that. All human life, and therefore their actions can be seen, - as you said from the start, - as meaningless and without inherent meaning.
I would like to claim that this exists on two planes, in the greater picture, yes, all human action is completely irrelevant and meaningless, since nothing we can do will ever change the fact that we will all eventually perish. When we talk about the universe we humans are so insignificant that it is hard to grasp. We are but a tiny parasite on a tiny host in a teeny, tiny part of the whole, nothing we can do will ever have any major change on things. We will live as parasites on this Earth until we either destroy it and perish/go on to find a new host, or find a way that will not make the planet inhabitable so we can keep living here.
If we take a look at these things, for us humans they are huge; finding a new planet to live on, saving the Earth from our self-induced doom. Huge things, yet completely insignificant to the universe we are a part of.
However, like so many other things, what is and what is not relevant is merely subjective terms. Now I tend to disregard most discussions about subjective things as utterly pointless, because as long as there are two sentient beings in this universe there will be two opinions about it. This brings me to my next point.
Even if all our actions in the long run will be completely meaningless, that does not change the fact that what we do today will have an impact on the life of people living now as well, and as long as we have impact on the present, our actions will carry some kind of temporary relevance.
I wouldn't say that this temporary relevance is 'fake' per se, but I hope you will understand what I mean here.
Relevance, importance and meaning, - none of these really exists beyond a personal level, no subjective thing really does, so as longs as we don't act these opinions out, they are completely irrelevant to everyone. I can sit at home and have the answer to world hunger, if I don't voice this then it's completely meaningless, right? But if I voice it to someone who listens, will it gain meaning?
I would like to say that for a while, it does, but in the end it doesn't matter, because even if I made sure no one went hungry, they'd eventually die anyway. And this goes all the way up to the top. It doesn't matter if we managed to invent proper space travel, it doesn't matter if we developed into superhumans that live for hundreds of thousands of years, eventually we'll perish anyway.
I'm sorry if you find that this were completely incoherent.
TL;DR: To try and summarize this fever-induced clusterfuck of a rant I'd say this: Yes and no, on a personal level, everything we do carry meaning, but in the larger picture, nothing ever matters.