Can someone who knows more about Eth tell me if this staking is a 100% guaranteed to happen? Is there a chance that miners rebel or a fork is made?
The miners could keep running the old PoW chain but it's unlikely to have significant value. Exchanges will support the PoS chain and some are already offering staking services. Tokens collateralized by off-chain assets will use the PoS chain. Etc.
With the PoW chain having little value, miners will be deeply unprofitable and most of them will have to quit. Moving to other chains isn't much of an option because other GPU chains have little aggregate value.