I would chose the one that means the most (maybe that's the whole company party, maybe it's just your department party or maybe it's the party where they hand out the Christmas presents/bonuses!), and go to that one. After that, skip the rest. I mean, how many parties does one company need to have anyways?? It's like gift giving nowadays; everyone's going to extremes, trying to outdo everyone else. Even Christmas trees for homes are reaching record numbers and record sizes. Now every affluent middle class person seems to think they need a "formal" tree and a "family" tree--at least. It's getting ridiculous.
As for answers to give people when they notice you're not turning up or that you don't eat when you do turn up, decide if you want to tell them the truth or just lie. You might even have fun making up wild stories just to see them race around the office gossips like wild fire.
Truth: "You know I have IBS." "This all looks great, but I'm afraid it's too rich for me." "I have to be very careful about what I eat." "Man, I wish I could have some of that [insert best looking item]. It looks wonderful! But my stomach is just not cooperating today." "I'm afraid I can't go because I am feeling a little unwell."
Almost a truth: "I have food allergies." "I've gone totally organic for health reasons, so I'll have to eat the lunch I brought for myself." "I'm on a diet." "I've been down with the [horrible catching disease] lately and I don't want to pass it around." "I can only pop in for a minute; I just have too much work to do to take time off."
Wild and crazy lies: "I'm Jewish." Even better, "I'm a Jeohavah's Witness" (they don't celebrate or even participate in any holidays, including birthdays, so that can get you out of a lot). "I am now a strict atheist and just can't celebrate a religious holiday in any form or fashion." "I'm a pagan and Christmas ripped off ancient Goddess worshipping rights, so while I have no problem with you celebrating your holiday in the way that you see fit, I'm going out at the Solstice and offer up greenery to the god of the sun. Want to join? The women will be performing an ancient fertility dance in the nude." "I belong to PETA now, so I'm a total vegan and can't eat anything that touched something that an animal product touched because that's just not fair to animals." "My [insert relative] just passed away, so I'm not feeling too cheery" (this one can be repeated multiple years since the anniversary of said loss makes you unhappy). "Me and [significant other]... well... I don't want to talk about it right now. Just please excuse my absence." "Oh, is the party today? I forgot all about it and made my doctor/dentist/mechanic/hairstylist appointment and I can't cancel because [so-and-so] books months in advance!" "I'm afraid I can't go out this evening as my car is in the shop and I'm having to ride with a friend." "I'm boycotting the party because they won't let me bring my "life partner" as my spouse." "I can't come; I'm working a second job right now for the money so they don't foreclose on the house." "I'm afraid that conflicts with my new class on exploring my femininity and inner power" [launch into tirade about how "the man" is keeping you, as a woman, in a subserviant position, etc. ultra-radical feminist dogma just to keep whomever from ever again asking you to another party]. "[Whisper] I can't go to the party; you-know-who in [department name] keeps hitting on me and the guy's a total weirdo! I don't know how many more times I can brush him off. Don't talk to him or he'll start asking you to hook us up!"
Of course there are more, and they can get even wilder and change from day to day. See how long it takes people to figure out you're making it all up. And then laugh like it was a big pratical joke. When asked what it was all about, say it was just to get back at the gossips for a year's worth of backstabbing.