1. Everyone absolutely has to use a service to get their coins on a forked chain.
Why does everyone have to use a service? Who would run the service? Would the service need to use a service? If the service doesn't need to use a service, then nobody has to use a service, because they can all do whatever the service would have done.
Althuogh theoretically it doesn't look like it but conceptually this is a change in the protocol, because without this someone's old bitcoins don't work.
What do you mean "someone's old bitcoins don't work"? There's no reason that old bitcoins wouldn't work jsut fine as long as there was enough hash power supporting that blockchain.
2. What if I double spend? I.e, I try to do the same on both chains simultaneously?
That's not a double-spend. That's two completely different spends on two different blockchains.
Waiting for confirmation on other chain won't work, since it could lead to a deadlock & would require a semaphore to resolve which means requiring coordination between the chains.
I don't think you understand how bitcoin works. Please avoid explaining bitcoin's functionality to new users until you've learned a bit more about the technical details.