Optical Soundbar for Center channel speaker?
Soundbars are built to diffuse sound all over the place, so are probably the worst possible choice for a center channel speaker ever in the history of audio. Also there is a difference in arrival to speaker time between digital and analog sources which would reduce even this to a horrid gabble. A small center speaker, or just a small regular speaker would work so much better. Unless your receiver has a pre amp output for the center Chanel (not speaker level) you cant use it for that anyway.
No. Read Lance's answer.
probably not...The problem with trying to do it that way is that the direction of the signal is from speaker output to optical. There are very few converters that do analog to digital...Its common to go from digital to analog but the other way around almost never happens, You might be able to find some off brand converter from china that would work in that direction but even if you do you will be facing latency where the lip movement on screen will not match up with the voices coming out of the speakers...Some AV receivers do have some compensation for latency but my feeling is it will be too severe and you will not have enough compensation available in the direction needed to achieve a satisfactory result...another problem is that most of the off brand converters process line level signals not speaker output so additional conversion might also be needed to process speaker output to line level output, this additional processing will only exasperate the latency problem.