I think that I understand you.
Personally I think that "Additionally" in text of Night indicates that the text after is not alternative, that applies when Doors of Night is in play, to "Environment. The prowess of each non-ranger Dúnadan is modified by -1.".
Example of alternate (not stacking) effects that depend on presence of Doors of Night is the text of The Sun Shone Fiercely.
The Sun Shone Fiercely wrote:Environment. -1 prowess to all Orc, Troll, Dwarf, and Ringwraith characters not at, nor moving to or from, an Under-deeps site. This modification is -2 if Doors of Night is not in play. Cannot be duplicated. 'Go back to the Shadow! You cannot pass.' -LotRII