Before you tie yourself in this knot it might be useful just to look and see if there was a Roman census in that time period:
"When I administered my thirteenth consulate (2 B.C.E.), the senate and Equestrian order and Roman people all called me father of the country, and voted that the same be inscribed in the vestibule of my temple"[0]
That Rome did censuses and kept detailed records of the censuses is not in dispute. The thing that never happened is people making long trips to the ancestral lands.
The entire point of a census is to get an accurate population count for reasons of taxation and public spending. People uprooting to go to grandpa's home to be counted messes with that count. It's counter productive. Rome would never have required this and in fact would have tried to restrict travel during the census because they wanted an accurate population count.
The much more likely explanation is the author of Luke needed Jesus to be born in Bethlehem, which was problematic because Jesus was well known to be from Nazareth.
That doesn't describe a census or anything like it. There is absolutely no evidence there was a census covering the Roman empire let alone the whole world (as actually stated in gLuke).
However, there was a census of Judea ordered by Quirinius when Herod Archelaus was kicked out in 6AD. And that makes sense because, prior to that time, Judea was a client state so Rome would not have directly taxed it. Once it became a province, it would be subject to direct taxation and, hence, would have needed a census to determine the taxable population.
So, by far the most likely scenario is that the author of gLuke was referring to this census but got his facts a bit wrong. He made way bigger whoppers than that one.
"When I administered my thirteenth consulate (2 B.C.E.), the senate and Equestrian order and Roman people all called me father of the country, and voted that the same be inscribed in the vestibule of my temple"[0]
[0] http://classics.mit.edu/Augustus/deeds.html