Yes, I think we should have the free-threaded CI to be on par with old GIL CI. So “yes” to all your three questions.
The PEP 703 acceptance makes clear that it’s experimental to begin with, but we definitely want to ship something with 3.13, and it’s only six months until the beta freeze.
We have no idea how long each of the three phases will be, but community involvement and feedback is essential for progress, and for success. The more we can include in each release, the better, then we can get more feedback and adapt, and hopefully shorten each phase.
This means we can’t really afford regressions in the free-threaded builds caused by “normal” PRs that didn’t run the free-threaded CI.
Finally, we’re expecting C-extension libraries to test on both free-threaded and GIL builds, so we should also experience the pain reality of double testing 
(I don’t think it will be too bad, but let’s find out.)