x is usually taken to mean a damped string used for percussive effect: probably better to use (say) ? for unplayable notes. Unusual to see tab taken from 'piano' music. Normal guitar notation uses leger lines for notes below the treble staff. By melody do you mean top notes, or do you designate the music in four independent voices which can cross?

Identifying tuning of open strings is good - can the program cope with modified tunings as this would seem to suggest?

Stu