For all intends and purposes, you can assume here that is true, yes. It's not theoretically, because there are more private keys than addresses, which means there are multiple keys that result in the same address. But that's not important. It's definitely not the case that you need multiple private keys for one address, and that one would only unlock a smaller amount of the balance.


I'd say so, yes... you haven't send any transactions recently to/from that address? It was completely untouched for a while (at least a few hours)?

Again, try importing the same private key into blockchain.info and look what that gives you.