I am not concerned. Just asking questions to find out how the system works and how I can make a practical use of it. Why my interest in ''what address the coins are on''? - if I ever wanted to conclude a contract to purchase a car with an even less Bitcoin-knowledgable user I would put my (the payer's) address in the contract so that the seller could not argue in the court that the address from which he received the money wasn't mine. This is my silly newbie's reasoning. I am not concerned - just asking questions.
OK, then it's fine. Questions are good, certainly better than rants
In this event of two addresses and two privkeys in two different Bitcoin_Qts, I am correct to assume that whoever of the two users / owners of such address, the winner is the one who spends the bitcoins deposited to the address faster? The other guy loses?
Yes. Exactly.
Why not? Because I might lose / damage such privkey or it might get stolen or for yet other reasons?
Because of the reasons I mentioned, such as inadvertent private key sharing. Use transactions to send coins to people.
What does it mean in practice / real life ''to export addresses as a means of sending coins''. I lack the jargon.
It means don't use "dumpprivkey". Ever. (I meant exporting privkeys, not exporting addresses)
My first guess (proved false by you) was that Bitcoin accounts could be used like this:
a) you have a wife and a child: from acc #1 you manage your operations, from acc #2 you manage your wife's operations, from acc #3 you manage your offspring's operations, there is also acc #4 for the lady your wife shouldn't know about
b) you run three stores: and you assign each account and addresses within this account to a particular outlet,
c) you travel a lot: you can safely use and report acc #1 in any jurisdiction, acc #2 is a secret one,
d) etc...
For (a) and (b) you can use them. That's exactly what they're for. They're just a ledger, so you can have say 20 coins in account A and 10 coins in account B. After spending 5 coins from account A there will be 15 coins in the account left... etc..
Apart from spending from them, you can move coins between accounts arbitrarily with the "move" command (as kjj mentions). This moves the coins in the internal ledger.
You can also receive coins on accounts, so if you create a receiving address for account B, give it to someone, they send coins to it it will be credited to account B.
What I was trying to say is that they have zero association with low-level implementation details such as private keys and inputs, and do not provide any isolation on that level. So if you want to do hiding like in (c) they are not the appropriate choice. You need separate wallets in that case.