It could be (and often is) argued that Mohism did succeed, since the "official" doctrines and ideas of the Han owe as much to Master Moh as they do to Confucius. I don't think we have good enough records to talk about precise PoDs at 400 BC, but the notes I've got say the Mohists during the Warring States period were organized, tightly knit, and fans of defensive warfare, coming to the aid of many parties under attack. If we assume the Mohists grow and thrive in that period, you may see a more slowly united (or not united?) China.
If (as seems likely) they fail to prevent the unification of China, you still have an insular, hierarchical bunch of people who disdain science and aesthetics, are big fans of wealth redistribution, and have an impressive martial branch that concentrates on defensive war. What you have, in fact, is something an awful lot like the Medieval Catholic Church. I can see tensions between the Emperor and High Priest mirroring tensions between Pope and Emperor in the West during the Han. When the Han fall, the Mohists may fall with them, as they don't seem to have coped very well with social disorder the way Confucianism, Daoism and Buddhism can. When the *Tang arise, they probably won't be Mohists anymore, although what doctrine they espouse I couldn't guess (probably a form of Buddhism, but not the same as OTL).
My two cents anyway. Do we have any classical Chinese scholars around here?