The origin of religion is a fascinating topic. With the discovery (or rediscovery) of Gobekli Tepe, it has become much more interesting. While many sociologist have take the position that culture and cities first arose, and then religion followed, the view is beginning to change. It may just be that religion was in fact first, and from that we begin to see the development of cultures and city life. This definitely would change quite a bit on the subject.
Now, while it is true that ancient cultures did borrow from each other, they also came up with many of the same ideas independently. The idea that there were 16 other crucified god-men in the ancient world really is not evidence that cultures borrowed from each other. In fact, Kersey Graves, the author of the work in regards to those 16 other god-men, simply was not a very good researcher (at least not on this subject). The work is even rejected by most who accept that Jesus was copied from other sources, mainly because Graves had his facts wrong. There are a couple god-men that he mentions that he most likely made up (as in, they are not mentioned anywhere else, or in any historical record), and his facts are hit and miss. Much of what he says is either wrong, or based on information that originated after Jesus.
Even the similarities between Horus and Jesus are minimal at best. In fact, if you look at an individual like Augustus, one will find similarities with other gods, as well as with Jesus. However, they fall quite short of being so identical to Horus, or any other supposed god-man.
Many of the rules and symbols, etc, did not need to be borrowed either. There are similarities between ancient Native American laws, and Ancient Hebrew laws. They obviously did not have any contact, but there are various concerns that many deal with. While there are some similarities that are definitely borrowed from one another, to distinguish which ones were can be very difficult. Some of the most recent work that I have been doing is on the influence of Zoroastrianism on Judaism and later Christianity. Some of the similarities are striking; however, the big question always has to be is, is there a need for the group to have borrowed such an idea? Or, is there any evidence of such borrowing? Often the answer is no.