(1) what should I do in order to save my bitcoins if something happens to blockchain.info. Can I access to my bitcoin with ANY wallet, if I know my address and private key, or if I know my address and password? if so, how do I find my private key?

You should backup your wallet, using one or more of the options blockchain.info provides.

(2) I've also noticed a backup option in blockchain.info wallet. If I use it, can I get access to my bitcoins with this backup, with every wallet? should I save a new backup after every transmission of bitcoins from my blockchain.info wallet?

With wallets that are compatible with the format, such as Multibit, yes. I think it's safe to say that there'll always be some tool on the Internet you can use to read the file, but downloading the Multibit client (even if you don't plan to use it right now) wouldn't hurt.

As for how often to backup, I'm unsure of blockchain.info's setup...if you only have one address, then you only have one private key, and you don't need to rebackup. If you have multiple addresses, then you need to backup periodically or even after every transaction. You can see if the backup file is any different to see if you actually needed a different export.

Blockchain.info has the following instructions for accessing your backup, even if blockchain.info is permanently offline:

• Download the Multibit desktop client available for Windows, Mac OSX and Linux.
• Launch the application. Choose Tools -> Import Private Keys.
• Click the "Choose Import File" button and navigate to your wallet.aes.json file. Change the file format to *.aes.json and open the file.
• Enter the wallet password and press unlock. If required enter the second password and press unlock again.
• Click The Import Private Keys Button to complete the process. Your balance will be restored once the blockchain has downloaded.

(3) Wha'ts the best way to prevent a situation where a malware "reads" my password when I enter it?

Make sure there's no such malware when you enter your password. One way to be sure this is true, is to work from an offline Linux Live CD. E.g. see How to set up a secure offline savings wallet and/or bitBuntu. You can decide for yourself, based on how likely you think it is that you're infected and how much money you're dealing with, whether that's worth the trouble to you.