I have a few questions about blockchain compatibility.

I think it should be possible to copy a whole blockchain into a new crypto currency, with all the keys and transactions, so that you can restore your transaction history and balances by importing your private key into the new wallet. Is that correct?

If yes, could one modify the blockchain to erase unwanted transactions and move balances into nirvana?

Another noob question: If a wallet has multiple addresses, is it possible to tell if they are belonging together or do they all have individual keys, or in other words - can you restore the actual wallet balances?

Is it possible to send coins using a secondary address, or will they always be sent with the main address?

Some more  off topic question about 51% attacks:
Afaik you can take over a chain if you have the longest one in the network (chains, i'm talking about chains!).
Would it be possible to speed up the calculation of the chain for such an attack by modifying the block time in the source code and then start calculating, so that you can override far into the past until the last checkpoint?

And finally - how do you trace such an attack AFTER it happened? Are there any traces that can be found in the blockchain?

Thanks for your replies