I am not an expert on Hebrew grammar and sentence structure, so I will leave it to the experts, some of whom you have already considered.  I have actually looked at this passage in detail because Jewish scholars who reject Jesus have tried to undermine the messianic aspect of this passage.  Here is what the Hebrew, translated directly and literally, in the original word order says:   Yes, they will mourn    they pierced   whom    on Me    they will look…       So, yes, I would say that, from a non-expert point of view, it is fairly obvious that the on whom they will look, after being pierced is “Me.”   To say differently appears to me to be an ad hoc hypothesis, intended by Jews to avoid the somewhat obvious meaning.  Please take this for what it is worth, but, as a non-expert, the simple interpretation is the “Christian” one, which the one who is pierced is the one who will pour out a spirit of Grace (earlier in the verse) on the House of David, and that he will be an “only son.”  That is the “Me” to whom the piercing refers.  This is rather clearly a messianic passage, with implications about how the Messiah died, and why he died.  Whether the Hebrew word dakaru means pierced specifically, or whether it is a more broad word about how Messiah would be killed (perhaps not literally by piercing) is a debatable point which I would not bet my life on, but the word pierced is justified by the word.