Crater lake doesn't currently have Rivers out or in. The water level is maintained by precipitation and evaporation - so you are right. Opposition from basically everyone. Death valley varies from outflow to inflow every 400 to 500 years currently no outflow iirc so if we want to wait a few hundred years... Interesting ideas though

I'm visiting near the Salton Sea right now and have been learning more about its fascinating history. I don't know about the technical advantages or disadvantages (evaporation?) but there would be minimal local opposition to such a project, that's for sure.