Are sequencers and provers in-house or external? How do you ensure that your sequencers and provers maintain decentralization?

Polygon zkEVM's sequencer will be centralized during the early stages, there is a roadmap to decentralize the sequencer in future releases. Likewise, the Prover is centralized at the beginning, but the vision is to enable a Provers market. 

Provers cannot do much but generate proofs. So to have a decentralized system is much more critical (and difficult) than the sequencer.