I hear ya wrt oysters, and seafood and crustaceans more generally. It has been many a year since I even went near any of that, after a particularly bad bout of food poisoning 
from crab which clearly wasn’t as fresh as alleged, and wife similarly subsequently with oysters (before she became a full-time veggie). Ne’er again, as they say.
Good wishes for next year. I presume that by Monday of Christmas Week you mean 22 December.
To be sure to get near to the Christmas food that you seek you will need to go shopping on the Monday you are arriving, Tuesday morning at the lastest. Don’t leave your shopping until Christmas Eve. However, even on Monday you will be too late to order your selection from the traiteur. Turkeys have been on sale in Lidl will be gone by then, at least that is the case where I live. You will find a chicken or a capon and some butchers will be able to provide you with a large piece of pork but I would ask when you arrive but don’t expect the skin on for crackling. I see someone is saying they will be on sale in Leclerc but that is not the case in my local Leclerc.
You state you have found vague and contradictory advice and you will probably find the same here. It took me a while to realise that just because one person says something is available in (for example) Leclerc, it doesn’t mean that it is available in EVERY Leclerc.
If you drink tea British strength then bring you own from the UK.
You will find food like Foie Gras, Smoked Salmon and larger supermarkets, or specialist shops will have a good range of fish.
The shops will all be closed on Christmas Day but open as normal on Boxing Day.
Even that’s not strictly true. At least one boulangerie will be open in our local town and in our local village, the little store will be open selling everything including BBQed chickens.
The bakery here that opens Christmas Day considers that as part of its vocation.
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