Why is it important to make sure that miners spend money (on electricity) while mining? To avoid changing the transacion due to the cost of chaging themc ompared woth the rewards that are collected from mining new transactions.
Why would it be very difficult for someone to go back change a previously added block? because all the blocks would need to be remined and rehashed again form the beginning together with the news ones mined meanwhile.
How does the network regulate mining difficulty? Lowering the target hash.