How to use the bitcoin client without downloading the blockchain? I would like to be able to use the bitcoin python client to be able to do things like sign transactions offline, create transactions, create multisig transactions, etc.

Is there a way to do this without having to download the whole bitcoin blockchain 20 GB?

I don't mind looking up the bitcoin addresses, outputs and such using an external website (e.g., blockchain.org) using some sort of API.

Is the a client that does this?
Electrum is an SPV wallet which has most of the functionality you mentioned.

Multisig is an issue though, the primary solution I'm aware of at this time is hybrid wallets such as Copay.