Best of luck, paddy. I have loads of sympathy for you as I had a Christmas two years ago when my DD was three months old which was punctuated by comments about
a) how often I was feeding her (er, when she asks, dingbats)
b) how come I was feeding her so often (er, her stomach is tiny and she gets hungry and I'm not here to refuse a hungry child what she needs)
c) why I wasn't winding her (because she's not windy, maybe?)
d) why I didn't put her in a bouncy chair/the moses basket/a bed/on the sofa/on the floor/etc and get on with enjoying myself (er, because I like holding her because she is my daughter and I love her and she likes it too)
e) whether she could have a bit of roast potato (no, because she's a baby YOU LOONS and she likes MILK)
f) why didn't I try a bottle (because breastfeeding was working out just fine and I didn't see the need)
Really, just stay as calm as you can, prime DH to step in on your behalf when needed, if anyone asks you something stupid just say 'well, I'm doing this my way and I don't want to argue so shall we just agree to differ?', and pretend you need to see to your son's nappy about twice as often as you really need to so you can go off and have a snuggle with him in peace and quiet in a bedroom or something.
Best of luck.
And I love the idea of in-law bingo. I might start that myself (the issue of the day in these parts is why exceptionally clingy DD hasn't been to stay the night with them yet).