If we want to take aging seriously, we need to first appreciate the problem. It reminds me how people used to think there was "the cure" for cancer.
I wouldn't be surprised there's a few "One Weird Trick to Extend Your Life by Five Years"s yet to be found, but to solve aging I expect to be much harder. It wouldn't surprise me much that the simplest "solution" to aging could end up being the brain simulation thing anyhow, because fixing up the biological organism across hundreds of years is simply that hard.
If there are a few of those weird tricks I definitely expect a period of excessive optimism where people conclude we're just another couple of such tricks away from the solution, when instead they are simply the first steps on a very long path.
The latter is probably closer. We can already induce extended periods of apoxyia in animals by replacing blood with cold saline without brain damage so a similar procedure should be possible to keep healthy brain tissue intact while scanning it. The necessary fidelity of the simulation is what we don't know yet.
Keeping a human being alive for 500 years means knowing how to do roughly the same thing to bodies in order to accomplish the major periodic surgeries necessary to replace failing/injured parts including healing the brain itself, so probably represents a harder technical threshold.
That happens every night I go to sleep and a fair portion of my atoms get swapped out by normal metabolic processes. My neural network changes as I'm unconscious or dream. I don't even remember all my dreams, but wake up with new thoughts/ideas sometimes.
We're always some delta from ourselves the previous day. I'm very different from the me of 10 years ago. The jump to a brain simulation may be a large change, but I could also tweak the things that didn't feel quite right or match with what I used to be like.
Resource constraints are certainly an issue; the fairest way seems like pre-allocating a subset of resources to each duplicate who in turn can do the same. Societal restrictions on creating a duplicate with too few resources would prevent effective slavery or abject poverty.
Scenario 2: Same as 1, wake up in new body, etc. But they also wake up the old body. "Hey, I thought I was going to be in a new body!" They point at the young "you" doing a happy dance. "We just wanted to show you that it worked. Congratulations. Now lie back so we can deactivate you." The other one stops his happy dance: "Let's forget about that update ten years from now."
We've absolutely been here before with vitamins, both the fact that they're simple molecules and part where we over-promise what they can do. (Example: vitamin C mega-dosing)
That's a weird statement to make, while in the largest part of the 20th century, most medicinal advances were made using small molecules. Aspirin is a very potent drug that had dozens of actual applications at least, yet remains a very simple molecule.