1) As individuals? As a collective? At all? I'm posting this in the NF sub forum but my inquiry is posed to everyone.
I find I am more fascinated by the theory of personality than actual people. I do enjoy observing people in general, it's something I've done since childhood and comes as second nature as breathing, I don't have to consciously think to or attempt to observe.

2) Moreover, what percentage of us do you think is actually worth saving/fighting for?
What decides this worth? Hitler put lesser worth on a broad swath of people.
The answer might 'obviously' be kindness or goodness, but it depends on the ultimate goal; and who decides what is the kind act to do?
That, and ultimately it is the people in power, with the most power who are most able to execute their visions of what is worthy and what is not- that will bring upon the actual end result than just sitting around seeing which option is the good, bad, or worthy or not. The winners become the 'worthy', and they will keep expanding their views.
With that in mind, I am content spectating as humans scatter in whichever directions they please, that they think is the 'worthy' one.

3) But, to be fair, our ability to do great good (read: life/consciousness/health affirming in a positivist, humanistic sense) is also a uniting commonality among our species--I'm just unsure as to how often this actually happens and what that says about us in an "are we salvageable and worth the effort?" sense.
Would good perishing, or the momentary absence of it, really make humanity as a whole, worth forsaking?
If only 'bad people' are left, surely they are happy still to continue as they please in a system that benefits them-
in which they would see the world as 'worth' continuing.

4) Bonus question: How common is a disenchanted, cynical, nihilistic worldview among "feeling (and particularly, NF)" types?
I can only answer for myself, but while I can entertain cynical, negativistic thoughts, my feelings (which I think is the point of this question) rarely ever follow.
I do not get gloomy or cynical, adversarial, or feel that humanity is doomed nor do I doom humanity.