No, just one block with unavailable
TxObjectdata.
Yes if the committee withholds a TxObject from a bonded prover, they will be grieved. In practice, provers will run a full node to make sure they have all the data they need before making a bid.
For a proposal to be appended to the pending chain, a supermajority of the committee should execute the proposal and be prepared to propagate all the associated TxObject to peers. So if a TxObject is being withheld, that means the committee is dishonest (or simply not executing proposals).
Depending on the final slashing design, the sequencer and/or entire committee could be slashed which sets a minimum cost for such an attack.
A MIN_PROVER_TIME can also be set to make decentralized compute competetive [2] .
I agree that a minimum proving time removes some but not all centralization vectors. I think that in principle, such a minimum time alters the race to who can get included on L1 the fastest. Large provers can work deals with L1 builders to get a persistent advantage. It also does not alleviate redundancy and wasted compute by the provers.