Why is the exit plan the most important clause?
Because it is the only one you need precisely when the relationship has already soured. A partner things are going well with will hand over whatever you ask for; one you parted badly with will negotiate, and without the clause you have nothing to demand.
It should specify what gets handed over — code, designs, documentation, access — in what format, and within how many days.
How do you test a partner without risking an account?
With a small real project with a date, not an artificial test. What you are watching for is not whether the deliverable came out well, but how they behaved when something went wrong, because in a real project something always does.
If you are evaluating someone with a project like that, we work White Label in that same format: something real, something small, with a date.