I don't have a problem with the theological point the use of December 25th as Mithra's birthday was for the same reason. The defeat of the light over the darkness. My problem is with the tendency of some people to deny that christianity borrows traditions from other religions.
God clearly warned the Israelites to avoid doing that-- not to intermarry, to avoid the "high places", the ashtaroth, the household gods, etc. of Joshua, Judges, etc. These men were faithful to God's Word to the point of death as they established Christianity(don't put new wine into old skins) . Therefore it is incumbent on Christians not to roll over and accept the thought that there was the degree of syncretism you suggest. As remote and obscure as your sources necessarily are, and as subjective as interpretation of history is, anyway, all that really remains is the agenda(just like reading NYT today.) Do you believe in the literal bodily resuurection, and the literal virgin birth , or is your Christianity too broad minded for that old time religion?