The problem is that 2.b. is blatantly false.

Nope. Felling bad about bad things only helps you to protest or punish people (yourself included) for the way things are. This is a very indirect form of "fighting bad things", and one which is largely ineffectual outside our evolutionary social environment. In the modern environment, protest and punishment are next to useless for accomplishing anything.

Do you really believe that someone who felt happy despite knowing about the state of suffering in the world would be more strongly motivated to reduce suffering than someone who felt a great sadness and a burning desire to stop it every time they thought about it?

You're begging the question here: what is the difference between "strongly motivated" and "burning desire"?

That is, you just said, "I think that there is more M in S+M than there is in X", where you haven't expanded X.

(Also, I'm not sure what kind of brain can actually experience "great sadness" and "burning desire" at the same time. Mixed emotions are usually not extreme emotions.)

It might be more useful to restate your question as "Will an otherwise-happy person have a greater probability of increased utility per their values, than one who is sad about the world's currently low utility?" My answer to that is an unqualified YES. Happy people make better choices about which actions to take, have greater motivation to act upon them, and better follow-through -- it is simply no contest.

if your moral system dictates, as most of ours do, that reducing suffering is currently by far the best thing you could do, and you actually want, unlike most of us, to follow your morals to their conclusions

Let us draw an important distinction between "morals" and "values". To me, a "moral" is a statement about what levels of a value we should protest or punish (e.g. by shaming/shunning), and it is of limited use outside the evolutionary environment (where others shared our morals and could be more influenced by our social maneuvering).

As Asimov put it, "never let your sense of morals keep you from doing what is right." The people who put a lot of moral weight (using my reduction of "moral") on the elimination of suffering seem much more motivated to protest, "raise awareness", "speak out", and perform other social signaling behaviors in place of direct action.

OTOH, I know lots of happy entrepreneurs who give or volunteer on behalf of various causes who, AFAICT, are not at all outraged or depressed by the suffering they witness. Without exception, the people I've met who actually DO things about the problems of the world (as opposed to merely talking about them) are happy people who are not disturbed by suffering in principle, even if they may be moved in relation to some particular individual's suffering.

So, in the context of this post, what is being said is that dropping one's moral rules allows one to pursue one's real values without impediment by our biased instincts to protest-and-punish -- which are in any case mostly ineffectual in the modern environment.

I wholeheartedly agree.