Regarding Combat Adds for Missile attacks, there is really nothing definitive to start with. The 7.5 rules are very vague about Combat Adds for missile weapons and judging by what Ken has mentioned in the following excerpt, that seems to be by design: Combat adds are the same for all weapons.
Combat adds and missile weapons - Page 68 (7E) or 93 (7.5E)
Whereas strength increases the damage potential when fireing an arrow with a bow or when throwing a knife or a chakram, strength should obviously not count towards calculating combat adds when using a firearm or a crossbow.
KEN'S ANSWER: "Strength has little to do with how much damage a firearm does in combat. Neither STR nor SPD should give firearm adds. What you might do, however, is give double LK and double DEX adds to those using a firearm."
I also asked him if missile adds still exist under 7E.
KEN'S ANSWER: "No, missile adds have gone away. It no longer seems logical to have them."
Therefore, when firing an arrow with a bow, you should apply normal adds (which depend on your STR, DEX, LK, and SPD).
~ Trollszine! #5, Summer 2012, "Reflections on the Tunnels and Trolls 7th Edition Rules" by Patrice Geille (Questions and Answers with Ken St. Andre)
This is how I will handle Missile Combat Adds in my T&T games.
All artwork in this blog entry is public domain
Combat Adds (Melee/Thrown Weapons): STR, DEX, LK, SPD
Missile Adds (Non-Thrown Weapons): DEX, LK
Thrown Weapons: Utilizing STR, DEX, LK, and SPD make total sense. The SR vs DEX is still required to determine if the target has been hit.
Bows and Slings: Once you pull the bow to its full length, any additional STR by the character more than likely would not have any additional affect on the damage done by the arrow. One could argue conversely with a sling and STR, but I will try to keep this simple (I know - too late for that). SPD could be argued here, but I believe missile weapons are basically a one shot at a time deal with T&T. Where in melee combat, SPD would give you more bonuses for getting more hits in, it would not translate to the same for these types of attacks. Using only DEX and LK makes sense to me.
Firearms and Crossbows: Once again, STR really does not fit with these types of attacks as it is not used at all through the course of the attempt to hit and the ensuing damage. SPD could be argued as well but I do not think it fits either. It would fit if it reflected the attackers ability to get off more than one shot, but I do not think the T&T combat is really taking that into consideration.