Ah yes, I think we can all empathize with that.
In Resolve, you can very quickly add a new version (Color -> Add Version) which will duplicate all of your work and allow you to start making changes. What's also great about this is that all of your tracking and keyframing come through as well.
For your example you could have had the first pass as Version 1. When he came back with the notes then you make Version 2. For the next round of notes where you're only cooling off certain shots, change those specific shots to Version 3. That way in the morning, when he said "I like these shots V1, these V2, these V3" you would have all of those grades ready to go - just pick which version of each shot before rendering. Also, taking the little extra time to label nodes when you have specific changes can really help you remember what the intention was.
If you want stills for each version, you can quickly make a new still album, and in your viewer right click - grab all stills. You may not have the exact frame you want, but all of your node work will be there.
Hope this helps!