How desirable is the separation of beacon blocks from execution payloads w/o the other guarantees that epbs has been associated with? (unconditional payment, builder protection w/o relay) How much value do we get just by ensuring that an execution failure still leaves a valid beacon block?

It is valuable but it's not inherent to ePBS, this is a feature of execution tickets too, so imo this value should be weighed against alternative designs which achieve the same feature.

Robust Incentives Group @ Ethereum Foundation

https://rig.ethereum.org

I think we can have this feature by itself, without the whole epbs machinery. I think we can even do it in a way which is compatible with future more radical redesigns that separate consensus and execution slots (like execution tickets or regular epbs itself)

It seems as though the possibility of valid beacon blocks with invalid/missing execution payloads would somewhat mitigate the impact of execution failures so worth doing