Alan, you are spinning yourself dizzy asking so many questions and not properly reading the responses and/or getting too many responses from too many people who are talking about wildly different things.
You do not have to focus with a filter in place if used on the FRONT of the lens.
For any filter BEHIND the lens you must focus with it in place.
I seriously recommend less questions and more shooting. You seem to ask the same questions over and over and over and are just confusing yourself. And, most any of these question can be answered with a simple test with your own equipment. Have you noticed your images with filters on the front of the lens noticeably out of focus? There's your answer.
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