A bit late, but the only point where the cruelty play a role during the Pale Rock assault is for the bridge
(need to be at least 6 as you also said), it’s only one point down, really.
In fact, each of the Strength, Charm, Potency, Cruelty, and Mercy are tested with the expectation to be of at least 6.
Well, the Strength (that is the second easiest acquire) have an alternative that just skip the test (as mentioned in your 11th point).
So in my case, since I kept my Potency and Cruelty to 0, I got 2 points down.
This also mean that the maximum of points down from this event is 5.
The points up are from sparing on the assaults on the Abbey, the Cove, the Farms, the Fort, the Town, and the Wildlands during Act IV. By two for the Town. Then an additional one each for Wolfmen and Mercs if recruited. Then on that is added the remaining treasury.
Meaning we can got up to 6 to 8 prosperity points.
So your "It is definitely much lower than it could be.", yeah… not that much on this regard.
Then we remove the points down
(that can, as said above, go up to 5).
For example, in my case, I got 5 points up
(since I decided to not bother with the Wildlands and the Abbey, will do next time for two extra points) and 2 points down
(as mentioned above), meaning a total of 3 prosperity points at the start of Act V.
So, what really matters is not how much Cruelty we have, but how much we conquered and spared before the final assault, because otherwise it could be zero points down but with zero points up (aside from the treasury, Wolfmen, and Mercs points).
That being said, with all the morale its forces had:
And that only the Cruelty was bellow the required value and that the Wolfmen were recruited, one could ask themselves why the prosperity is not
6 + 1 - 1 = 6.
Well, the answer is as explained above, not enough spared territory. Would wager the 3 comes only from treasury.
