Lots of questions here
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A teleconverter designed for the FD mount should operate with the aperture in the lens in the same way as the lens itself works. One exception might be an older teleconverter, designed for an FL lens, which I would assume would work with an FD lens, but would necessitate stop-down metering.
Meters in FD mount cameras don't actually know what aperture you've set. They just know that the aperture you've set lets in 1, 2, 3-3/4 or whatever number of stops of light then the maximum aperture of your lens. (The EF lens Canon cameras are different - they stubbornly want to be able to tell you the exact number of the f/stop set).
So the meter reacts to a 2x teleconverter exactly the same way it responds to a 2x neutral density filter - it sees one stop less light, and instructs you accordingly.
Teleconverters add all sorts of distortions/aberrations, in varying amounts. Chromatic aberration, spherical aberration, pincushion distortion, barrel distortion - they are all possible. The distortions/aberrations will also depend on the teleconverter/lens combination - for example, a lens with a moderate amount of pincushion distortion might be a perfect complement to a teleconverter that adds barrel distortion.
Some teleconverters are designed for close focus work - and some of them are limited to close focus work, while others (most I would say) do permit infinity focus. As in most lenses, teleconverters often give optimum results with particular lenses, at particular subject distances. That is why you see teleconverters from the lens manufacturers that are described as being for use
only with certain lenses.
I have and use a Vivitar macro-focus teleconverter with my Olympus OM equipment. It gives acceptable results at infinity, but really shines at close distances.
One final point- I see from your signature line that your longer lenses are zooms. In my totally unscientific opinion, teleconverters tend to work better with fixed-focus lenses.
Hope this helps.