I've tried to give feedback/answer the 5 questions:
1) walletpassphrase YOURPASSPHRASE 10. This I beleive unlocks the wallet for 10 seconds.
correct
2) During the 10 seconds, type dumpprivkey. Does this leave the bitcoins intact in Bitcoin-QT wallet? Does this dump a file showing all the private and public keys. If so, what is this file called and its extension?
Bitcoins aren't something physical, they are not stored on your pc. If you "own" bitcoins, it basically means you have access to the private key that can be used to create a signature for one or more unspent outputs that funded the address that was created by hashing the public key belonging to the private key in question.
In very simple wording: your bitcoins are just lines in blocks that are stored on thousands of computers. Importing or exporting private keys doesn't change anything (unless you use an unsafe procedure/PC and your private keys get exposed, in this case you might get robbed)
So, dumping the private key will not "destroy" your bitcoins.
dumpprivkey only dumps one private key to stdout. If you need to dump all private keys, you can use dumpwallet (dumpwallet actually takes a parameters in which you can define the file that will be used to dump all keys to)
3) Use importprivkey to import private keys back into Bitcoin-QT. If so, what is the procedure.
open the debug window => importprivkey --privatekey--
This procedure will rescan the blocks on your disk, importprivkey takes an optional argument that will avoid this rescan (if you want to)
4) What does importmulti do? Is this relevant?
I've never used this, but apparently it's used to import multiple private keys at once
5) If this works, is there still a problem that if some bitcoins are sold, the 'change' bitcoins are still lost?
I have a hard time understanding this question. IF you use a decent wallet, it will guide you trough the process of creating a transaction spending one or more of the unspent outputs controlled by you. A decent wallet will make sure it funds the address of whoever you're sending BTC to, and it will make sure the change is being used to fund a change address.
Only if you manually create a raw transaction, or if you use a newbie-unfriendly, bad, wallet, or if you start messing with options you don't understand, you might end up losing your funds.