Good questions Simon and after a couple of years in Webflow, I still come back to them.
The design tool step has pros and cons. You create it without hitting html / css technical hurdles. You also create some of your design assets like shapes, masks etc that you can bring into Webflow.
On the other hand for fairly simple sites or where you have one you are going to blatantly copy, wireframes/ webflow feels like enough.
I keep my Webflow sites in my account so they are in my dashboard. I wouldn’t if Webflow had capabilities where a client could add me as a “Designer” collaborator. I believe client hosting should be in their own name. I hope Webflow add this functionality in line with other Website platforms. Most of my clients are local organisations I have an ongoing relationship with. I wouldn’t do this if I felt it were likely to be one off job.