That's incorrect.
Legally, the DECLARATION of someone's death is reversed. That doesn't change nor touch upon the facts or definitions of death.
Medically, if it's reversible they didn't die. They may have erroneously have been declared dead, but much in the same as above, it's that declaration that is reversed. The facts or definitions of death are untouched by this.
None of these have bearing on what constitutes "death" in a legal sense.
Wrong. This is a common misconception arising from metaphorical use of "death" in the vernacular. "I died during surgery" "I died on the table" etc. are purely conversational tropes that we employ for dramatic effect. They are not medical terminology per se, have no medical or legal meaning, and are NOT employed in the context of resuscitation (again, outside of common parlance for shorthand or dramatic effect). If you get resuscitated then you were never dead, by definition.
Neither does it cover apparent death due to e.g. hypothermia or tetrodotoxin poisoning (some cases of which made it all the way to autopsy before being discovered), except metaphorically (same as above).
You don't want that. Because then all we have is him literally saying "I was legally dead". That's the only SURE thing we have. Maybe he lied, you say? OH YEAH, MAYBE MAYBE MAYBE
I assume you don't mean "legally dead" which is very much not that (as you yourself have pointed out, someone can have been declared legally dead and then have that declaration reversed) but "legally speaking, dead means irreversibly dead"?
That's a problem in the MCU, because how do you KNOW? We know people can get resurrected in the MCU years after they died. It happened. What if, say, all the people who died over the last, say, 200,000 years all come back to life in 50 years because Dr. Strange screws up something? Does that mean they're currently not dead? It would have to mean that, because they COULD get resurrected, which means their deaths ARE reversible in principle, even if they're not actually reversed in practice. And that creates a lot of legal problems, because you can't just put all legal claims in abeyance while someone is dead, then resume them if and when their deaths get reversed and they are no longer dead. That would create a massive undue burden on the legal system - what about inheritance, contracts, criminal charges, etc. Do they just pause for everyone, just in case their death wasn't actually permanent? And if you DO stop things on what you THINK is irreversible death (even though it might turn out not to be), then why isn't Mr. Immortal off the hook for the same reason?
That depends what you mean by "survive". One may well argue that he in fact does NOT survive, he just comes back from death. Same problem as above - it requires a lot of legal fine-print and interpretation to get right. You can't just gloss over it with a word like "survive" when the law is involved. Every detail matters.
Because we don't know MCU laws. We know OUR laws, but we don't know if they work the same. And we also don't know if our laws as they are written would, in fact, apply that way to someone like Mr. Immortal, if he really existed. As I've pointed out several times, there exist a multitude of possible legal arguments he could make against this. Whether or not they'd be successful in a court is another matter, but he could at least make a case for it. It's far from simple and obvious, because he breaks the a-priori assumptions of many legal definitions.
It also involves "common medical consensus", usually. This is a necessary prong in the definition because what is and isn't "reversible" has changed historically, and is likely to keep changing. You could easily demonstrate to a judge that something like e.g. getting decapitated would indeed fall under "common medical consensus" of death. And then Mr. Immortal could just do that.
The law lives and dies with details. Those are of great importance in jurisprudence, and they can change outcomes dramatically.
You're creating something that doesn't exist. Reckless endangerment is a crime, as is forgery (usually). Whether or not faking your death is involved in it is immaterial. And as I said, those could well be past the statute of limitations by now, depending on the exact circumstances. Or could be technically crimes, but too difficult to prosecute to ever bring to trial (e.g. because it's too long ago, because you can't find witnesses, because evidence has been destroyed, etc.).