Without more information, I’m going to guess that if we actually save Saint-14 permanently, it’s because his death took place inside the Infinite Forest. Osiris’ machine probably only works in there. Since Cayde died outside the Infinite Forest, the method we may save Saint-14 with probably wouldn’t work elsewhere.
Red war wasn’t in the forest but cabal are using it to change the outcome
That’s because Mercury actually has a shit ton to do with both the Vex and the Red Region. We’ve seen ‘em there no few number of times either, but let me remind that the Tangled Shore/Reef is all the way on the opposite direction from the sun and pretty much lacking in Vex altogether. If they had any reaching influence over that far, they’d definitely be showing there with some of that architecture.