It seems to assume that all the information that constitutes the brain function that defines some one can be physically preserved. What if some is in the form of signals that require the brain to function continually to be preserved?
This. We don't know if a brain keeps all information necessary for a "reboot" or not. It may decay over time if whatever machinery necessary to first "start" consciousness is never used after birth.
I've used some psychedelics recreationally, and as far as I can tell, at least my consciousness is definitely affected on an ongoing basis by whatever signals happen to be in the brain. It's some sort of feedback loop and I'm not so sure how easy it would be to just start up again if it ever ended.
Simulators may have to just guess, and the simulated brains could end up with an altered consciousness compared to when they were alive.
We do not even know what consciousness is. I am not even sure there is an agreed upon definition that would fit the layman's understanding of what consciousness is.
The hypothesis that consciousness is a by-product of brain/nervous system/body activity is, in its current state, just this: an hypothesis. Not only that, but it is pretty much an hypothesis outside of the field of modern scientific models, which are based on a materialist methodology (assuming that everything has a material cause), such that we do not really have mature ways to study anything else. The materialist ontology ("there is nothing else than matter") only developed later in history, once the methodology was so established it became accepted as fact, rather than just as a (very successful) method of working.
It might be polemic, but there is also data from the study of NDEs and memories of past lives in infancy which, in my opinion and that of many others, should at least motivate us to stay open to the possibility that consciousness is something more than a by product of brain activity. Some propose that the nervous system could act as a form of "receptor", in the same way a radio receives a program. You cannot experience the radio program without the device, but it does not mean that the radio "creates" the program. Obviously those theories are no less wild guesses than the consciousness-as-byproduct ones, but they show that other models can have internal validity and cannot just be rejected because "obviously the cause must be in matter".
I could go on and on, but to people interested in going down this rabbit hole, I always recommend the report of the Galileo commission, and detailed call by academics to open the width of interpretations of consciousness that are considered acceptable enough to be studied: https://galileocommission.org/
All of this to say: we do not know anything about consciousness, so any such business is just preying on rich people afraid of loosing it all or unwilling to contemplate their life decisions, as one feels drawn to when the end feels close. It is sad and absurd.
(Before I get burned to the altar of scientific wisdom: I did not say that the theory of consciousness-as-a-byproduct is absurd or should not be studied. It is good to test it and try to make it work, and I have deep respect for the researchers in this field. I just argue that, in the current state of our science, there is no reason for it to be the only acceptable theory)
This is unfortunately not a definition of consciousness, rather an hypothesis about the cause or origin of it. Consciousness is an experience, as you mention in your next sentence:
> I also believe in brains that have more than one consciousness, as I've personally experienced a similar thing before
This is what makes it so tricky. My only definition of consciousness is pretty much "I know it when I see it", with the annoying caveat that I cannot see any other experience than my own...