It is absolutely true that the SA boycott only grew on the back of the uprising, and a generalised black struggle against apartheid. But the thing is, they then helped to feed each other. The boycott (or the proposed boycott) helped to keep the struggle in peoples minds - if you are sat around going 'its shit but there's nothing we can do' then the story drops off the agenda, it means our government are less likely to even try to do anything. The boycott helped to force the government to make condemnatory noises, and some (very minor) acts. That's a help.
Of course it wont (and didnt) win all by itself, that will be down to more local working class forces (the wider arab w-c, especially, at the moment, in Egypt), but anything that adds pressure is of assistance.
As to the jewish and arab w-c uniting in Palestine itself, well, I still see precious little hope for it, or examples of it. Even in this weeks Socialist - which has a generally very good article on Gaza - it doesnt offer much hope, cant point to any particular examples of such unity in action. Sad, but true.