How about using NUnitForms? You can write a test verifying that clicking a button opens a window, then filling a form and closing the window makes the record appear in the main form. Quickly make it pass, and then refactor it to the MVP pattern or whatever you find appropriate, adding tests as necessary. So, after you are sure it works, you have plenty of time to think about responsibilities and such, and if you don’t add too many tests, you’ll be able to redesign later.

In addition, NUnitForms makes it very simple to fire UI events. You can avoid subclassing your form, for example, and other hacks.

One reason why I usually avoid your approach is that you have to keep all your pieces in your head at once. Note that several times you have to make a mental note in order to return to it later. This way, if you forgot something, all your tests are green, but the thing can’t do anything useful. But that’s probably me, I don’t like making design decisions before coding.