You have to trust the code from bitaddress.org.
When you import the private key into Multibit, it could take a while. I'm not that familiar with light clients, but it may require downloading and parsing the full chain.
Could someone please confirm whether or not downloading the full chain would be required to import a private key into Multibit?
Don't forget the change. Let me give an example. Say you backup the private key for address A, you transfer 100 BTC to address A, you import the private key, you spend 10 BTC which causes the 90 BTC change to be sent to a new address B, and then delete the client on the USB stick. If you were using bitcoin-qt, you would have lost the 90 BTC. Again, I don't know what Multibit does.
Could you (or someone else) elaborate on this please? From my understanding what happens when I spend 10 out 100 BTC from wallet A, the remaining 90 BTC don't stay in wallet A. The client automatically creates wallet B and sends the 90 BTC there; as a result if the client is deleted withough wallet B being backed up, the funds are lost, right?
It seems that to counteract that, all I would need to do is create paper wallet C, and transfer the 90 BTC there before deleting the client. This way I don't need to bother with any backup, and the 90 BTC remain safe and offline until I need them. Do I understand this correctly?