I have had a few theories for jon . first I thought he was the older brother of Dany who was born premature to Lyanna , thus killing her in the process. unfortunately that would make Jon much older than Robb so a non-starter as you say. I had hoped they were twins but GRRM says jon is 8 months older so no-go . I then found the theory that Brandon and Lyanna may have hooked up but I was told the timeline does not add up. I liked that one because it would explain why Brandon was so upset at the crowning of lyanna by rhaegar. I don't think Ned is the father because he never thinks of jon as his son . Now I tend to believe Arthur Dayne may be his father but that may be confirmation bias on my part since I am convinced beyond a shadow of a doubt that Mance is Arthur . I agree Lyanna has to be the mother. if I think of some more I will get back to you. thanks for listening.