Don't know how you come to that conclusion, it has never been God's message that Christians should obey the laws of the old testament ... Jesus summed up the requirement for Christians as 'love God with your whole heart, soul and strength and your neighbour as yourself' ... Christians are not required to obey the Mosaic Law but the principle that Jesus stated above encompasses all anyway, e.g. if you love your neighbour then "thou shalt not steal" is unnecessary as you don't steal from one you love.
a) What about them?
b)Did you read the genealogy in Matthew and Luke? It proves the lineage from Jesus' parents back to Adam, the first created being ... the Jews were looking for the promised Messiah and they could check a valid claim (there were many) from the lineage amongst other things. For the Jews the record of family lineage was a vitally important matter and kept up to date until Jerusalem was destroyed by the Romans and all records were lost, hence no Jew today can trace his/her lineage back past that time.
As with all things related to the bible record, great care was made to ensure that facts were correctly recorded and documented by the official Scribes, who were responsible for recording matters accurately.