Just an update:
I'm pretty sure I have mandolin and guitar sorted on that aspect but I have got mountain dulcimer wrong. It appears that md tab is "upside down" when compared to guitar tab (position of the lowest note in tab) and is handled differently in the program - I had not taken that into account with my "fix"...
I have also noticed that abc2tab does not appear to handle chords or unisons correctly. 2 notes at the same time, e.g. [AD] would produce 2 notes one after the other and I feel that could be misleading.
I don't know what the effects of that would be with the dt or with sources I would use, if wanting to use tab (mostly dance music). It could well be a rare occurence.
As for fixes, I have decided that they are best left to the programmer. It is probable for example that I have mucked up the md tab by not understanding the differences between lowest note highest line tab and visa versa.
It is also true to say I don't understand Perl well enough or understand the programming logic fully. Also, even if I got the Perl right for Mandolin and Guitar, I'd have to invest around $50 to get a compiler that is not for a 30 day trial (and exe's produce a "trial version message") and to top that off, I'm not sure my programming is up to that... Writing a front end is easy(ish), writing a forum such as folkinfo only involves limited logic, as does making use of some of these programs but at least to me, these little programs we take for granted involve far more programming skill...
I'll try to help if I can and I'm very much in favour of GNU software and its development but I think I've got too much on at the moment to do much with this one (and in any case am uncertain about my own ability)... Perhaps one of the many able programmers here can help?
On a completely different area, I believe John's aim is free software that everyone can get hold of and to make it as accessible as possible so that everyone can view a quality print or get a tab for their instrument; ideas I very much applaud (and I think in keeping with a "folk principle") but...
On a commercial level, has anyone tried Tabledit? At first glance, it looks good to me and handles a variety of instruments, can import abc. etc. There is a free download but the product I think is around $50 to register.