I feel the Chainmail description for the Warg leaves alot to be desired, but I'll attempt to throw out my views on the matter.
Mass Combat Tables:The Warg alone attacks as LH and moves as MH. It's reasonable to infer that he defends as HH due to the Wight reference, but I'm not sure this is necessarily correct. Though I wouldn't argue the issue.
Q: What benefit does the Warg receive when used as a mount? I will give two answers.
A: none. In fact he is penalized in movement to HH. The benefit is to the rider who is mounted. In the case of a HF Goblin, he now becomes MH and gains the charge movement rate of HH. As a consequence, the Charge Morale rules will apply also.
A2: If it is insisted that the "two attacks as men" reference is applicable to the Mass Combat system, a Goblin mounted on a Warg would not fall into the category of MH or HH for attacks. Instead it would be counted as 2 HF (but move as HH). I say HF because when the term "men" is used, it usually implies "foot men". In contrast, most of the other descriptions are specific in presenting the fighting abilities as "foot" or "horse".
MH vs HF get 2 dice per man/ 5,6 kills.
HH vs HF get 3 dice per man/ 5,6 kills.
HF vs HF get 1 dice per man (so 2 dice)/ 6 kills. This is actually the same as the Warg fighting alone as LH. LH vs HF get 2 dice per man/ 6 kills.
Just to note, jacars approach of giving a mounted Warg +1 would produce LH vs HF get 2 dice per man/ 5,6 kills. This is similar to MH, except when opposing AF, MH, & HH. Not a bad option.
Man to Man Tables:The description says, "In combat against fantastic opponents give them two attacks as men. They require a score equal to that necessary to kill a Wight to kill them."
What this tells me is that Wargs get 2 attacks on the MtM table (this would be against either "normal" or "fantastic" opponents). As I mentioned above, the Battle of 5 Armies article classifies Wargs as 2 daggers, leather armor. When in melee with a "normal" figure, they would get to roll
at least twice on the MtM table each round (the rules on page 26 for classes of weapons would apply). To hit the Warg would require a roll between 5-8 or better on 2d6 (depending on the weapons class) if you used these weapon and armor classes for the Warg.
When the Warg is used as a mount, it would not receive an attack until the second round of melee and it could only attack footmen. But it could attack a different opponent from it's rider.
You suggested that "normal" figures could not attack "fantastic" figures and this is not the case. There are some that are immune to "normal" attacks, but some are not. A Warg could enter 1:1 melee with a Hero, for example. This is why I think they used the "Wight" to describe it's defense value. So, the Warg could attack a Hero on the MtM table, but it must make 4 simultaneous hits in a
turn to kill the Hero. In contrast, the Hero could roll 2 dice on the FCT as against a Wight. A score of 6+ will kill the Warg. What this does is put the Warg into a category seperate from "normal" figures and the "fantastic" figures found on the Fantasy Combat Table. It's a different way of distinguishing the Warg in the frame work of Chainmail, without including it on the FCT.
I'm open to others interpretations on this matter though