I suggest we hold a council:...

Now that's more like it :)

1. Double Strike is no different from an unretaliated strike... A x2 coefficient would work for both...
2. Dragon Strike should be lower than multi-strike and "biting," because it hits a maximum of two units, while the others hit at least three. Archers who use area-of-effect spells (liches and magogs) can be equated to multi-strike.
3. Using mathematical expectation is the most accurate way to calculate coefficients, but the essence of the issue isn't so fundamental as to require bringing out the heavy artillery. The reason is that we still won't account for all influencing factors, and the margin of error will be much higher than the precision of the tool.