The means have nothing to do with the ends

In Chapter 11, Ramchal makes an amazing statement. He says that just as someone who steals a Lulav gets no merit (and actually a sin) by shaking that Lulav, so to someone who steals time from his employer to do a mitzvah gets no merit from having done that mitzvah.

This statement does not differentiate between mitzvot bein adam l'Makom or bein adam l'chaveiro.

In the case of a stolen Lulav, since shaking the lulav is a mitzva bein adam l'Makom, it makes sense for Hashem to say "Well that lulav is stolen, so I don't want your mitzvah." But if someone is a Robin Hood, and steals money from one person and gives it as tzedaka to a needy person, even though his act of stealing may have been wrong, why should he get no reward for having given charity to someone in need? At the end of the day, the poor person had enough money to put food on the table that night!

I think what you see here, is that our obligation in life has nothing to do with the actual outcome of our actions. It is our obligation to always do the right thing, and it is up to Hashem to actually make sure that those actions have proper outcomes. If you give charity to a poor person who accidentally drops it down the drain, you still gave charity, and will be rewarded accordingly. And if you steal from one person to give charity to another, even though the charity was given, you will get no reward.

I think this is the idea of "mi shebikesh la'asot mitzvah, v'neenas, mekabel sechar keilu asa'ah - if someone tries to do a mitzvah, and for some reason beyond his control he is prevented from doing so, he receives reward as if he had done it." Divine reward and punishment are not tied into whether one actually accomplishes something (physically), but rather, whether one does their absolute best to do the right thing.