Yes, there's a lot of factors controlling the HR. The list is too long, but for our purposes we can focus on heart stroke volume, blood volume/thickness (that's dehydration, too), blood distribution dynamics and adrenaline stimulation (ever noticed higher BPM in/before a race?)
Yes, your heart have been improved and you don't NEED to hit your max anymore.
No, your node have not changed much. Your true max hr is still about the same. It might have lost a beat or two due to the aging process.
One caveat here: we are talking about normal healthy people. It is quite possible for the "node" to malfunction or even to stop working. In fact, I had a problem with this node diagnosed in my youth when it was shown to have "migrated", whatever that meant, resulting in arrythmia. There's a fallback mechanism built-in in the heart where other group of cells takes the role of heart's pacemaker or you could stimulate it externally (electronic pacemaker, etc) and achieve a higher max. Theoretic limit in this case is about 300bpm and there were even cases of higher than 400 in real life.
Think of it like a modern CPU which use a frequency generator. You generally don't run it at full frequency, unless the load requires it. And you can overclock it, sure, but that's another story.