The placement of string/fret is another weakness in ABC2TAB. It only works out open position. The commercial program, Tabledit seems a lot more advanced. I think it uses rules to try to work out a logical fingering pattern. Of course you can always edit the tab.

Interesting idea about the program - I for one would very much like to see it if you ever do write it.

I think that the closest current abc program to your requirements is the Mac only Barfly. That one does work with text containing abc. An example of where it works well is Jack Campin's modes tutorial You can read through the whole document and when you get to a protion of abc, that tune is displayed as "dots" and you can also play the tune. I think it also imports and/or exports to musicxml.