Hi Jochen,
Good that you are trying out your wallet backups with small amounts and testing whether it works.
There is a description of all the files that are created by MultiBit on your disk that you might find useful here:
https://multibit.org/en/help/v0.5/help_fileDescriptions.htmlI thought I would describe how I backup my wallets as you might want to do something similar.
I have a few Truecrypt volumes at various locations that I use for backups.
The top level directory is:
walletBackups
Then if today I want to backup a wallet called 'savings' I would create a new directory:
walletBackups/2013-11-02
and in there I would copy:
savings.wallet
savings.info
savings-data (and all its contents)
I don't delete earlier backups (They are only small. You do not want to delete the wrong thing by accident)
This way I can open the file savings.wallet directly in any copy of multibit and check everything is ok.
There is no single 'best' way to do backups - I know some people rely on TimeMachine and other people use automated cloud backups.
In your last paragraph I did not quite understand what you are trying to do. MultiBit is at
https://multibit.org and blockchain.info is a separate wallet entirely. You can migrate a wallet in one to the other (as described in the MultiBit help) but generally you only want to do this if, say, blockchain.info is being DDOSed or perhaps you don't like MultiBit and want to move your private keys out to another wallet.
Generally I find copying private keys around 'promiscuously' leads to confusion and errors so I recommend keeping things simple.