Mohammed, thanks for your comments, appreciated! In terms of controversy regarding Seneca, I think things have settled at this point, as we are fairly confident that what scholars consider his corpus of works was indeed his. The only doubt is the Pumpkinification of the Divine Claudius, but even that one seems settled at this point.

Regarding Stoicism and Cynicism, they certainly did influence each other, but Cynicism is (slightly) older and we have more evidence of its influence on Stoicism rather than the other way around.

Finally, Buddhism. You are correct that there are a number of similarities between the two, especially concerning the ethics. But I don't think anyone has established any direct link either way. We know that Pyrrho the skeptic went to India with Alexander, and that he talked to local sages, but Pyrrhonism is very different from Stoicism, and even there it is hard to tell whether there was any direct influence.