I would leave a couple mL of headroom, do not fill to the brim. No, VG or PG will not expand in the freezer, it will contract a bit, if anything. But if your filling area was cool when you fill the bottles, VG (not sure about PG, I don't use PG) will expand if the room you bring the bottle into is warmer than the room you filled the bottles in originally. I had one 100 mg VG that I filled in a cool kitchen in January, and brought out into a hot office in August, and it was filled to the brim. Started leaking out of the cap and down the sides of the bottle. Yikes! So now I always leave a bit of headroom in the storage bottles.
I would not bother with the CO2 purging. It will not hurt anything, but there is O2 naturally dissolved in VG or PG anyway. All VG nics I've had store as is in the freezer indefinitely. If the nic is really pure, so does PG-nic, but if it is a less pure nic in PG it may age some in the freezer. I had this with TW 36 mg Red Label unflavored, which over the course of a year or so turned into essentially a tobacco juice. I think nic-oxides impart a tobacco-like flavor. I could not vape it myself, but the person I gave this stash to treats it like gold, from the aged flavor it took on. So oxidation is not necessary a bad thing, but again, if it is very pure nic, even PG will not change significantly in the freezer.
I use 30-50 mL glass bottles, since a 50 mL bottle of 100 mg will last me months, and I don't want to continually be dipping into a large bottle. I keep the bottles in a sealed pet food container in case of breakage. I use amber glass bottles from SpecialtyBottle.com, the ones with eurodropper insert, which is excellent for safe syringe dispensing and protection from spills will the bottle is in use. They also limit air exchange greatly when the bottle is open. they are essentially an inert plastic septum (HDPE which has no plasticizers and does not dissolve/react with the contents over time, like polycarbonates or PETs).
I developed this method over three years ago, and I still have some of my VG-nic from then in the freezer. It is virtually unchanged, and I expect preservation to be indefinite. Actually this has become a bit of a problem, since the quality of unflavored nic has increased a lot on the last few years, and some of what I have in the freezer from 2009 I don't particularly care for now, compared to some of the current nics. But that has nothing to do with storage effects, everything to do with the original nic quality.