Basically, the rules for both "Backstab" and "Ballista" are wildly different between editions. An answer that applies to one edition would be totally inapplicable in another. You're asking a question that would have several equally-good answers, which is bad on this site.
Now, if you changed it to be specific to an edition, it would be perfectly answerable.
Another alternative would be to specify which editions you care about, and asking for an answer that covers each. Like, if you mostly played 2nd ed, but you also wanted to know how it worked in ODND and AD&D, then asking how it works in those specific editions would probably be focused enough.
This site works best when asking specific questions about actual problems. What problem are you facing that can only be solved by knowing the ballista/backstab rules in 40 years and ~10 editions? Do you really want to know all of them?