And who decides that?
Your brain does. After you actually die, it's irrelevant, unless you believe in an afterlife. But just as you make your attack, you gauge your odds and act so as to ensure you will either be the victor, or if the opponent is worthy, that you will die a battle-death. In order to be capable of this, you must of course live your whole life venerating the warrior spirit and cultivate this mindset of seeking a battle-death and avoiding a foolish death. Thus you become a manly and honorable person, which is why the concept of battle-death is held in such importance across many cultures - it is a vital mechanism of promoting healthy male gender roles.
You said earlier that if you die of wounds years later then it's not a battle-death.
I didn't say that, must be someone else. All torpedos share ID, you know. If you want my take, then if you didn't die, there's no problem. If you killed the guy, great. If you ran away or were incapacitated, that's cowardly and/or shameful, but no matter - you're still alive, you can fight another day. If say you were crippled, spend next several months in bed, and then died in your sleep - that's a shit situation, and such deaths from injury or disease were traditionally feared by warriors, which is why they were eager for battle and would rather charge into melee and risk death, than be rescued by healers and risk becoming cripples with no chance of a good death.
Or maybe you'd slip and fall and die anyway.
Probability is not changed because an improbable event happened. Winning the lottery doesn't become likely because you just won. Of course, history is how we estimate probabilities, but it is important to understand that probabilities exist a posteriori, and changing our estimate of the probability is not the same as the true probability changing. In relation to the warrior spirit, a warrior shouldn't be entirely clueless, but accurate statistical estimation is IMO not vital for a warrior. It is however vital that whetever he feels the odds to be, he respond to them in a manly way, so as to ensure that if he does die, the death will be a battle-death. And on the topic of feelings, I think the brain is actually really good at these estimations, it takes all that stuff you said into account. Of course, if you are raised right and have discipline, it is of course a lot more accurate. But ultimately a man need not second guess his instincts on what precisely the odds are - the important thing is to strive to act such that if possible, he is the victor, but if he is to die, the opponent must pay dearly, or at least risk his own hide.
Actually, ancient people did not consider it brave to charge into a hopeless situation, but stupid. Note, when I say "hopeless", I don't mean not having hope of survival. To a warrior, his own death is of relatively minor significance. I mean hope of doing your enemy harm. Which is why the Spartans at Thermopylae are revered as heros, even though they went into a battle with no hope of survival: They did have hope of killing many Persians.
Not if you didn't see him.
It's not about if you saw him, but if you could have. If it's night and he's got the best NV gear but you have night blindness, you're fucked, you need to GTFO there because if you get shot it will be a pathetic, shameful death. You need to use your brains to avoid such situations, and resist commanders that put you in them. One of the commander's duties is to provide a battle-death for his casualties. If he fails in this, he doesn't deserve to command.