I generally take a test bud from somewhere in the middle. However, I would say that your plant is way too far away to be taking test buds. I really think that you probably have at least 4 weeks left to go. If you start taking test buds at this point, when you KNOW that you have a lot longer, you are going to end up with nothing when the plant is mature. I will only take a test bud when the trichs look ready.
No, it will not regrow what you clipped. In fact this is a "wound" to the plant and it will try and heal itself, taking energy away from the development of the buds. This is a big reason that we do not cut things off the plant once it has been flowering for over 2-3 weeks.
I know that it is hard, but try and be patient. Harvesting a plant with mature bud will absolutely be worth it, both in terms of yield and potency. Your plant will bulk up a lot from where it is now.