How would this work in practice? Solvers give attestations to txs they see?
Attestations are one way (in particular to vet that settlements don’t execute any malicious interactions with the funds stored in the settlement contract), but even just having governance token holders run a some client we proveide to detect misbehavior and launch a slashing proposal in the DAO could be an option.
Separating solver responsibilities like you suggested is an interesting thought (which is more related to our vision for https://forum.gnosis.io/t/gpv2-road-to-decentralization). One reason why we wanted to have each solver also run independent infrastructure is that - unlike in year strategies - solvers don’t run on Ethereum but in some local setup (AWS, self hosted, etc). By having fully independent solvers the protocol should be more resilient (in the past we have seen that e.g. a node misconfiguration on our end caused a downtime of the entire protocol).