When importing, all private keys are transferred onto your computer from blockchain's cloud correct?
It depends on the file you are importing, but generally, yes.
(can only a part of the wallet be transferred?)?
It takes some effort, but yes I think you can export individual private keys from blockchain.info if you want to for some reason.
So the new balance in your blockchain account would be zero.
No. Importing keys does not change the balance in other wallets where those keys also exist. The balance in your blockchain.info wallet would remain unchanged until you started sending bitcoins from (or receiving bitcoins to) MultiBit. Depending on exactly which addresses you are using in MultiBit you may (or may not) see the balance in blockchain.info remain in sync with the balance in MultiBit, but eventually you would almost certainly start to see differences. It is a bad idea to try and use an address in two different wallets at the same time. Once you import a private key to a new wallet, you really should stop using the old wallet, otherwise the inability of the two wallets to keep synchronized will at best cause confusion and at worst lead to lost bitcoins.
Hence this transaction amounts to simply sending money to a wallet address from my blockchain.info wallet correct?
Importing private keys does not create any transactions. If you don't need to maintain the same bitcoin addresses in the MultiBit wallet as you had in the blockchain.info wallet, then you would be much better off just creating a transaction in blockchain.info and sending your entire balance to your MultiBit address.
Could be anyone, just so happens that in this case it’s my MultiBit wallet--but the idea is the same?
Which idea?
The blockchain wallet public address remains the same, only private keys transfer to Multibit and I can continue to deposit and/or receive BTC at existing blockchain address?
Importing the private keys means importing the addresses since the private keys are used to create the addresses. As I've already said, it is a bad idea to continue to use the existing blockchain.info address after you've imported it (with it's private key) into MultiBit.
Besides being what I call "warm storage" (on the computer, rather than in the cloud or cold storage), are there any other major (or minor) differences between online wallets and Multibit? Can I use Multibit in much the same way I use blockchain.info, or are there significant operating differences? I see the send/request tabs in Multibit for example—do these work just like any cloud wallet?
There are some technical differences in how change is handled. Also blockchain.info will allow you to attach a message to your transactions that they store seperately in their own database. MultiBit has no such functionality. MultiBit is known to occasionally get out of sync with the blockchain and needs to be reset to see your actual balance. Blockchain.info will mail you a copy of your wallet occasionally, with MultiBit you are responsible for creating your own backups.