I think the idea behind SGX based cloud computing would be that you send encrypted data and code to the enclave, which then decrypts the data, does the computation, encrypts the result, and then returns the encrypted result.

The enclave gets to see everything that's going on, but in theory nothing can view or corrupt what's going on inside of the enclave.

I'm personally pretty skeptical that SGX is a secure platform, however if you do decide that SGX works and is trustworthy, it would be possible to outsource your computation in a mostly cost effective way to anyone who has an SGX enclave.

Yes, this is the idea. Enclave technology should improve over time, whether it's SGX or something else. And Golem is working with Intel, Texas A&M and others in creating an environment called Graphene (https://grapheneproject.io/) where applications can easily be run on SGX.

Enclave defeating technology will improve over time as well. Things like scanners and lasers and emi detectors get better and better just as technologies for hiding from them do.