You seem to have misunderstood my hypothetical interpretation. The choice comes in with "may", not "when".

Option 1: You choose to roll any number (up to 5) of power dice with the attack. In that case, you override the normal number of dice you would roll, and the other effects follow.

Option 2: You don't choose how many dice to roll; you calculate the dice in the standard way (looking at the monster card, and adding any dice from effects like Doom). The other effects of the ability don't apply, because they are written (sloppily) on the assumption that you exercise option 1, similar to my "feint" example.

That's certainly not the only possible way to interpret the text. And it does involve the assumption that the writing is somewhat sloppy: like the feint example, the later sentences are written under the assumption that you exercise the option in the first sentence, but don't actually state that.

But there is no problem with adjudicating identical dice differently; either you replace your regular dice with "bash dice", or you don't. Your interpretation doesn't do well under the assumption that you choose and then apply modifiers, either: that would imply that you can't roll less than your "bonus dice", and also that the modifiers could put you above 5. Even assuming that you can't get around normal dice maximums and that the last sentence of the Bash ability is an exception, rather than a clarification, that would still prohibit rolling exactly 1 die when two copies of DOOM! are in play, which strikes me as rather silly. So I think we can safely assume, whether Bash is optional or not, that the "up to 5" replaces your total power dice, not your base power dice.

And your interpretation also assumes the writing is somewhat sloppy:

• The first sentence says "may roll up to 5 black power dice" (implying that "roll up to 5 black power dice" is optional) when it ought to say something like "chooses any number of black power dice to roll, up to 5", or at least replace "may roll" with "rolls". (Under your interpretation, you always "roll up to 5 black power dice", and so saying that you may do so is, at best, like saying that you may obey the laws of physics.)
• And then the second sentence says "any of THE power dice" when it should just say "any power dice".

As is so often the case with Descent rules, we are forced to assume that there are at least minor errors in the text no matter what was intended. Therefore, neither possibility can be ruled out merely on the grounds that it's not entirely consistent with a strict reading.

Antistone said:

Ok, I think I understand that better.
I still don't see how option 2 comes into effect. As explained below, no matter what, you will roll up to 5 power dice with the attack (Bash or not), so the may can only reasonably be referring to the choice of number of dice, not the choice of rolling those dice at all.

Put another way...
You are assuming the may applies to 'the choice' (act of choosing). "You may choose to do X" (use Bash)
I am assuming the may applies to the 'up to' (0-5). "You may choose the value of X " (number of power dice)

Since AFAICS every attack will always involve 0-5 power dice, your reading of what the 'may' refers to appears to me to be nonsensical. There is no choice involved. You will roll up to five dice on this attack no matter what you choose. Therefore that isn't the choice, so the choice must be the variable given, the number of dice.

Antistone said:

Which is why I think that that can't be the logical application of the choice. I am assuming that - it appears to me to be an immutable fact in Descent. Therefore it isn't a choose-able choice. Therefore it isn't the choice referred to by the may .

I think it is a fair thing to say that you do always roll up to five power dice. There is never, under any circumstances, an attack that does not have 0-5 power dice!
The may is therefore clearly appropriate only to the number chosen, not whether or not to 'use' Bash.

Or am I so twisted up on a sleepy post-lunch session that I've missed something enormous?