What is the best way to preserve oyster stew?
If the stew is already prepared, you should be able to freeze it with very little problem.  I freeze cream sauces all of the time.  Individual portions are ideal because they freeze more quickly with less/smaller water crystals. 

If the stew is not cold already, chill it in the refrigerator before before moving it to the freezer.  Breaking, or separation of fats, usually occurs when reheating.  Defrost the frozen stew gently, say overnight in the refrigerator or in a cool water bath, and then gently warm in a pan -- not a microwave.  Add a little fresh cream to smooth it out if you need to.

As you mention, any oysters in the stew will be frozen and reheated, so they will be end up more "well done" than those in a fresh batch.

As for freezing the oysters in their liquor, you can do so.  If you have a vacuum sealer that allows liquids, that is your best route.  Note that freezing will slightly change the texture and they will release more moisture when thawed.