The problem with unwritten traditions is that, because they're unwritten, they can be changed by anyone to suit their own prejudices, so this particular argument is entirely worthless.It's not unwritten tradition, though. It's using Scripture in the New Testament. Jesus and the apostles set out pretty clearly quite a bit on what doesn't apply anymore, and that includes the whole "unclean foods" thing, as seen in Peter 10:9-16.
EDIT: It doesn't matter if it was from St. Paul. He is considered one of the Apostles, and his written word is also considered to be divinely inspired.
I would expect that Jesus would counsel thusly: abortion is not inherently immoral, but throwing away human life because you can't be bothered to use birth control is. It's been shown that poor people who have access to birth control and to abortion services are better able to improve their lives and escape poverty than poor people who don't have such access. I cannot believe that Jesus would require women to carry children to term whose presence would place an unmanageable burden, financially or health-wise, upon those women. It doesn't make sense according to my understanding of Jesus' teachings.
(I'm not entirely sure if the people of his time understood when conception was. Pregnancies aren't even detectable for the first month or two without a pregnancy test, since many women occasionally experience a missed period that has nothing to do with sex. Consequently, I would expect the Bible meant not the literal creation of a zygote, but rather the point in time when a pregnancy would be confirmable without pregnancy tests, where it used the term 'conception'.)
I don't recall seeing anything at all in Jesus' teachings that advocate the destruction of a life because it might cause undue hardship. In fact, we're outright promised we're going to have hardship in life. If we use that logic, we should also kill older people that have become a burden, or those that have been born with physical or mental challenges, and so on. I can't see Jesus entertaining that train of thought. After all, the dignity of life is not in our physical or life circumstances, but how we handle those circumstances, and use them, regardless of our comfort, to give glory to God. (I am legitimately curious as to the verses you think would indicate as such, though! I'm always fascinated by interpretations of Scripture that are not my own.)
As for understanding conception, again, I don't think it matters if we're discussing in the context of the Word of God being absolutely true. As Christians would consider it to be the divinely-revealed Word of God, it does not hinge on the actual understanding of the recorder (check out the book of Daniel, and how confused he was with some of his prophetic visions). So if it says conception, it means conception, and I'm sure that's the reason, for example, the Catholic church has their particular dogma about birth control. I would fall into the Protestant camp, so birth control that prevents fertilization is okay by me, at least by that metric.