This can continue until the accelerating expansion of the universe places any other galaxies beyond our reach

If dark energy is constant, and if no-one artificially moves more galaxies together, then after 100 billion years, all that's left in our Hubble volume is a merged Andromeda and Milky Way. On a supragalactic scale, the apparent abundance of the universe is somewhat illusory; all those galaxies we can see in the present are set up to fly apart so quickly that no-one gets to be emperor of more than a few of them at once.

It seems no-one has thought through the implications of this for intelligence in the universe. Intelligences may seek to migrate to naturally denser galactic clusters, though they then run the risk of competing with other migrants, depending on the frequency with which they arise in the universe. Intergalactic colonization is either about creating separate super-minds who will eventually pass completely beyond communication, or about trying to send some of the alien galactic mass back to the home galaxy, something which may require burning through the vast majority of the alien galaxy's mass-energy (e.g. to propel a few billion stars back to the home system). There will be periods, lasting billions of years, in which some galaxies are beyond material reach, but visible and thus capable of communicating.

This cosmology of alien paperclip maximizers fighting over a universe of mutually receding galaxies, is not exactly proved by science, but it would be interesting to see it fleshed out.

If your civilization expands at a cubic rate through the universe

You're picturing the far-future civilization as a ball, whose boundary is expanding at a constant rate. But I think a more plausible picture is a spherical shell. The resources at the center of the ball will be used up, and it will only be cost-effective to transport resources from the boundary inwards to a certain distance. If the dead inner boundary expands at the same rate as the live outer boundary, we'll be experiencing quadratic, not cubic growth.

each couple of 2 has exactly 2 children

says a poly...

LW in general seems to favor a very far view. I'm trying to get used to that, and accept it on its own terms. But however useful it may be in itself, a gross mismatch between the farness of views which are taken to be relevant to each other is a problem.

It is widely accepted that spreading population beyond earth (especially in the sense of offloading significant portions of the population) is a development many hundreds of years in the future, right? A lot of extremely difficult challenges have to be overcome to make it feasible. (I for one don't think we'll ever spread much beyond earth; if it were feasible, earlier civilizations would already be here. It's a boring resolution to the Fermi paradox but I think by far the most plausible. But this is in parentheses for a reason).

Extending lifespans dramatically is far more plausible, and something that may happen within decades. If so, we will have to deal with hundreds or thousands of years of dramatically longer lifespans without galactic expansion as a relief of population pressures. It's not a real answer to a serious intermediate-term problem. Among other issues, such a world will set the context within which future developments that would lead to galactic expansion would take place.

The OP's point needs a better answer.

That all sounds rather well, but sort of lights warning lights in the back of my head - that sounds suspiciously like an Applause Light.

My current issue would probably be regulation - it's possible this will all happen within a century. What if some superintelligent beings don't get to handle it in a few billion years? What if WE have to face it, with the less than rational people wanting to live as long as possible (We might want to too, but try to look over the consequences. Or I HOPE so at least), then what? I'm not asking what anyone else should do. I'm asking what WE should do in this situation. Assuming it first becomes available in developed countries to those that can pay for it, and gradually, which to me seems most likely, what happens and what can we do about it?

This can continue until the accelerating expansion of the universe places any other galaxies beyond our reach, at which point some unimaginably huge superintelligent minds will, billions of years later, have to face some unpleasant problems, assuming physics-as-we-know-it cannot be dodged, worked around, or exploited.

Due to my innate, if misguided, belief in the fair universe, I hope that everyone can get their own baby universe to nucleate at will. The mechanism has been proposed before:

The bubble universe model proposes that different regions of this inflationary universe (termed a multiverse) decayed to a true vacuum state at different times, with decaying regions corresponding to "sub"- universes not in causal contact with each other and resulting in different physical laws in different regions which are then subject to "selection", which determines each region's components based upon (dependent on) the survivability of the quantum components within that region. The end result will be a finite number of universes with physical laws consistent within each region of spacetime.

All these "unimaginably huge superintelligent minds" have to do is to control this process of bubbling.