Combined Arms and Squares attacking difficult terrain

The rules state that unit and card bonuses cannot increase a square's dice above 1, nor can they increase a cavalry unit's dice above 1 attacking a square. If a French Old Guard (+2 melee) unit in square attacks a town hex (-2) may the unit use its bonus to offset the terrain effect whilst still rolling no more than 1 dice? A similar situation occurs not infrequently with heavy cavalry (+1) attacking an infantry unit in square uphill (-1). I think the rules as written are a little ambiguous but I think the intention is that no unit bonuses are allowed at all and both of these attacks would roll no dice.

In a similar vein the rules state that a combined arms attack may proceed even if the non-artillery unit involved has zero dice. What if this unit has (nominally) negative dice such as a 1-block infantry or a 2-block light cavalry attacking a town hex (-2/-3)? Is combined arms attack allowed in this situation, and if so must the negative dice be subtracted from the artillery's dice? I would argue that if your unit is not strong enough to reach zero dice then combined arms attack is not possible.

Apologies if these questions have been discussed previously.

as an off-the-cuff response, I would say that
1. Squares roll 1d unmodified by anything except terrain.
2. Units can go to zero dice, but can never roll negative dice, and ergo a combined arms attack in such circumstances = 0 + artillery dice. 

I agree with Mark McG.

The rule book states on page 17 (my emphasis):

The maximum number of battle dice an infantry unit in square may roll in melee combat is 1 die. Command card additional dice and troop type additions will not increase the number of dice a square will roll. Terrain modifiers, however, may reduce an infantry unit’s battle dice. An infantry unit therefore may not have a die to roll.

An infantry unit in square when ordered as a Combined Arms Combat, will add to its 1 battle die the artillery’s dice from the Combined Arms Combat.

Note, "An infantry unit therefore may not have a die to roll."; this is before the possibility of adding dice from artillery. So, agreeing with Mark again, Infantry may add zero dice but not negative dice.

Finally in "Infantry Squares Attacking" on page 18, it does not contradict the above rules; they apply just the same.

 

'The infantry or cavalry unit determines the number of battle dice it will roll in the melee combat including any terrain reductions. The artillery unit determines the number of battle dice it will roll including any terrain reductions. The artillery’s battle dice are added to the battle dice of the friendly unit making the melee attack. All the dice are rolled together at the same time. There may be an occasion where the infantry or cavalry unit battle dice result is zero. The artillery unit engaging in combined arms will still roll its determined number of battle dice. ' - p.18 3rd Ed.

Using the interpretation you suggest a 4-block light cavalry will have zero dice attacking downhill into a town, and so will a 1-block light cavalry, both providing the same degree of support to an artillery assault. I think that the wording is ambiguous but, given the strong dependence of firepower on unit strength in this game, I would be surprised if this is the intention.

I understand your point of view. It seems a little curious that a weakened CAV has the same battlepower dice as a complete one and give the same bonus to the supporting ART. Maybe a special situation unseen during the testing phase.
Merry X-mas from Le Mans - France

Merci pour votre reponse et Joyeux Noel de l'Australie