Hi there. Total newbie questions, I've done lots of googling & youtube-ing about bitcoin, but am not clear about the following issues.  Would like to be clear before I start trying to actually move any funds.  Could someone advise please?  Sorry about length of my questions.


I've installed the bitcoin-qt client.  
Say I have 1.5 BTC in my confirmed balance, that all came from one single receipt of 1.5 BTC.  So this is all on one address in my wallet.
Say I want to send 1 BTC to some address.
I enter the details to make the payment, incl. say a 0.0005 transaction fee for the miners, and send.  My remaining balance would be  0.4995 BTC.

I understand that when I send bitcoins to some other address, in the above case the client will send out the 1.5 BTC from my previous transaction balance, sending 1 BTC to the address I am paying, plus the 0.0005 BTC transaction fee, and send the change of 0.4995 BTC back to me, to a 'change address' that is hidden in the client.  

For security, say I am normally storing my wallet.dat in an encrypted folder on external USB sticks, and don't keep it on the hard drive, having created addresses in this wallet previously.  So I let bitcoin-qt client start up, and sync with network to catch up with the blockchain.  The wallet.dat that bitcoin-qt sees at this point is an empty one, the one it creates by default.

Now to make a payment I disconnect from the net, quit bitcoin client, and copy my backed up wallet.dat onto hard drive, overwriting the empty wallet.dat.  Start bitcoin client, bitcoin-qt sees my real wallet and knows my balance, I enter all the payment details, re-connect to the net and hit send.  Disconnect from net.  This means my actual wallet is exposed to an online machine for the minimum time, for security.    

Q1 - is best practice that I should now make a backup of the wallet.dat I just used, rather than just delete it and rely upon my earlier backups?   I'm thinking that the (hidden) change address that bitcoin created to arrange for my 0.4995 to be sent back to me will be one that my earlier backed up wallets will not know about, so would I lose the change if I don't make fresh backups every time I send BTC in this way?

Q2 - if I want to simultaneously send the 1.0005 BTC out, and at the same time clear out all remaining funds in that address by doing another transaction that sends 0.4995 BTC to another address of mine, is that possible right away, or do I have to wait?

I'm thinking here that my first transaction has effectively used up all my 1.5 BTC, and until the change comes back (i.e. until the blocks are created that confirm the transaction), I don't have any more funds to send?  Is that correct?  So I have to wait 10 mins for the first block, and longer for confirmations to be safe, before I can then send the 0.4995 anywhere?

Q3 - I am wondering why the default behaviour of bitcoin-qt client is that change is sent back to a hidden address?  I'm sure there must be some reason which I haven't understood.  Isn't hiding the change address going to result in a lot of ppl not realising it's in a hidden address, and what if they lose access to it through not realising they need to know the private keys of not only the addresses they have openly created, but the private keys of these hidden change addresses that the client has secretly created as well?

Thanks for your help in understanding the above.