Preorder Navajo Wars by GMT Games
Yet there are 6 outposts/forts on the board. Is the die roll to determine automatic victory mandatory?
Yes, the die roll is mandatory.
In other words, in this highly unlikely scenario, enemy morale would jump from 0 to 6 no matter what, unless you don't do the die roll. Which tells me you would be better off not reducing morale until you can get some of these buildings off the board.
If there are 6 Outposts/Forts on the map, you don't deserve an auto win (that's my design intent anyways). Your appraisal is correct in terms of gameplay: You need to do something about the Outposts before you make a lunge for dropping enemy morale.
Don't forget about the Optional Rule to 12.2. I actually prefer to play with this as it makes enemy bounce backs less dramatic.