what if a coin is spendable by multiple keys? it seems like the scripting language could allow multiple private keys to spend a coin?

what if i send a coin to someone, but also allow it to be spendable by me. would their wallet still show they received the $?
Depends on the wallet. To recognise the transaction at all, their wallet would have to have the script / multisig address in it too. They'd have to have done that themselves, you can't generally trick someone because their wallet simply wouldn't see the transaction at all.