To your point on being flexible and checking assumptions, I advocate founders to “fall in love with the problem” and clearly articulate why they want to become become experts on this specific problem.
This is great advice - I'm not a founder but I consult and one of the tricks to keep me motivated for each project is to make myself passionate about each project and what I can do. This might seem like circular logic, but becoming passionate about things is a muscle you can develop and it makes things so much easier!
I think it would be harder to challenge deep assumptions once you fall in love with a problem. It seems like all of your questions and answers would resolve around the base assumption that the problem exists, rather than questioning what the world might look like if it didn't exist. Instead of adding something to solve the problem in the current world, a small shift could eliminate it completely... and thus make the company obsolete, but the world better.
I find the most motivation when I hate the problem and I want to eliminate it from my life. Then it's not, "how can I make this better", it's "how can I eliminate this so I never have to hear about it or think about it again".