Fixed, almost always. You are buying an outcome, not my hours. Hourly shows up only in genuinely open-ended discovery work, and I will say so before the meter exists.
No. The retainer exists for firms that want it. An install with no retainer is a complete engagement, not a downgrade.
The scope names what working means before anything is built. If the system does not do what the scope says, I fix it on my dime. And if I do not think automation will work at all, you hear that in the audit, free.