No RAW it is twice as often 1 in 4 for 2d4 versus 1 in 8 for 1d8...
Hmm? Of the 16 possible combinations on 2d4, seven of them involve a 4, and one of those involves two 4s.
So if you roll a d8 sixteen times, you can expect to roll two 8s. If you roll 2d4 sixteen times, you can expect to roll eight 4s. You roll a 4 four times as often as you roll an 8.
Also how would you treat the critical d12's? Would all 6 need to be a 12 (~1 in 3 million chance) or would they be treated individally? RAW they would be considered individually.
Certainly. You roll one critical die per point of enhancement bonus. There are six of them. Each d12 can trigger a reroll.
You roll your weapon damage die once per [W]. The damage die of a falchion is 2d4. Each 2d4 can trigger a reroll.
The rule itself mentions dice plural for what plugs into [W], and the rule -itself- is not ambivalent in that regard.
Weapon Damage Dice: A [W] in a damage expression stands for your weapon’s damage dice.
I'd say the definition of Weapon Damage Die comes from p219, rather than p276, given that it's the Weapons page.
Damage: The weapon's damage gie. When a power deals a number of weapon damage dice (such as 4[W], you roll the number of dice indicated by this entry. If the weapon's damage die is an expression of multiple dice, roll that number of dice the indicated number of times. For example, a falchion (which has a damage die of 2d4) deals 8d4 damage when used with a power that deals 4[W] on a hit.
This seems to use the terms very consistently. "Damage die" refers to what appears in the column on the table, whether it's 1d8 or 2d4. "Number of weapon damage dice" refers to a plural of [W], not a plural of dX. "Multiple dice" refers to more than one polyhedron.
So the 'damage die' can refer to one physical die, or to multiple physical dice. There's no inconsistency of terminology on p219.
As you yourself point out, p276 is internally inconsistent. It refers to a single [W] as damage dice, yet we know that a single [W] can be singular in both concept and physical representation (1d8, for example, is one [W] and one polyhedron), so using the plural makes no sense. When p276 says "A [W] in a damage expression stands for your weapon's damage dice", it is misquoting p219.
-Hyp.