how different (in terms of distance) is, say, bottom Ab from G# on a violin?

Depends on what temperament you're using. The violin itself allows for many different ones.


I don't think I've ever seen such distinctions made on the fingering charts of wind instruments, though my experience is more with 'early' instruments than the more well-developed orchestral ones.

It was absolutely routine on the Baroque flute, and you still find distinct meantone-ish fingerings for Ab and G# in flute books of the mid-19th century. Quantz's book covers it; there are some in the charts in my "Old Scottish Flute Music" collection on the web.

http://www.campin.me.uk/Flute/Webrelease/Flute/Flute.htm

I normally use fingerings like that on the recorder - most descant recorders will give you a lower-register Ab with T12- 45-- and a G# with T12- 456-. An equally-tempered split-the-difference can require half-holing the 6. You get a similar choice between Db and C# on many recorders (though not all).