But...for someone who adamantly denies the existence of heaven, isn't it a moot point?
Yes, completely. I'm just curious of opinions.
Okay. Well, here's my roundabout answer.
The only things we as mortals can know about heaven, since none of us have seen it, are what the Bible tells us about it. And all we have in the Bible are glimpses. There simply isn't enough to go on to give a concrete answer. Not sure whether that is because (1) serving God is not supposed to be just about the "reward," so that's not where our focus should be, (2) we couldn't possibly understand it with our limited mortal understanding, even if we were told more, or (3) other reasons. But in any case, most of what we have are either fleeting explanations or, such is in revelation, symbolic descriptions. So almost anything we can say about what goes on in heaven is pure speculation.
What we do know is that it is a place where the souls of the saved exist eternally with God and with other beings. It seems that there is, as you indicated in your own post, free will. Those who are there seem to still be themselves, and have the ability to think, to know, and to act. With that, it seems to me, that the option to rebel exists. As you pointed out, Satan and some angels rebelled. As such, they were eventually cast out. It seems to me that the ability to rebel would still exist.
Although the promise of residing in heaven is described as eternal, the possibility of rebelling and being cast out is consistent with a relationship with God, because whether one is talking about the relationship of man with God in patriarchal times, under the Jewish covenant, or under the new covenant, we see the specifics of those relationships being different, but what is the same is that they are
always without exception, covenant relationships. In other words, they are always conditional. In a the most broad sense, the relationship is always, "you will be blessed endlessly
if you love and obey God." The conditional nature of that relationship seems to imply that the ability always exists for us to choose to break that relationship. If we do not, God will always keep up his end of the bargain, which is eternal paradise. If we do break the covenant, God keeps up his end of the bargain in that regard as well.
So, yeah. That's my take.