Can we really ever make any difference?

The following argument has been bothering me for a few years now:

1. The objective rightness or wrongness of an action is determined by the value of its consequences: an action cannot be wrong unless it leaves the world a worse place than alternative actions.
2. Nearly all of our actions have pervasive value-independent effects on the near and far future.
3. Nearly all remotely probable world histories that are consistent with the past and that diverge from each other in pervasive value-independent ways are on par with each other.
4. Therefore, most actions we can perform are not wrong.

This argument suggests an odd sort of nihilism; rightness and wrongness are beyond our capacities, not because of anything essential about rightness and wrongness, but because the way that events are linked up to each other leaves us powerless to have a positive or negative influence. If we commit a murder now, there will be a bit more local grief, but the effects of the murder will transcend our local situation, and will include both good and bad things. Different people will get married. Different people will be born. There will be a different number of artists, a different number of cancer deaths, a different number of lies told and relationships salvaged. All said, the local grief directly caused by the action will form only a small part of the action’s total consequences.

James Lenman suggested that consequentialists should, as a result, think that our actions have about a .5 probability of being net positive and a .5 probability of being net negative.  If the above argument is correct, however, there is a significant probability that the consequences of our actions are neither net positive nor net negative. Instead, they leave the world on par with its alternatives.

Is this argument right? It has been bothering me insofar as its implications seem tremendously important to our conception of ourselves and the moral situations we find ourselves in, and it seems plausible enough. But I have no idea how to evaluate the premises. In particular, while premise (3) seems plausible, it is not amenable to investigation by any traditional means that I know of.

As a result, I think that the right conclusion to draw from this question is that we should be basically uncertain about our capacity to make a positive difference in the world as a result of our actions. It is very likely that anything we do to make the world a better or worse place will have complex and diverse consequences that ultimately leave the world on par.