Ok, I'm rolling up my sleeves to help you. My family routinely hosts Thanksgiving for 25 - 30 and I have some advise for you.
Delegate, delegate, delegate. Really, the only things you should be doing are the turkey and making sure everyone has a place to sit and stuff to eat on.
1) Cook only 1 huge turkey in the oven. In addition to this turkey, buy enough turkey BREAST that you can prepare in a roasting pan. We usually do one turkey and one large breast (yes, you can buy just large breast meat) so you may need to do two additional breasts for your large group. You should cook the meat at your home. Your dad it right, your plan does not have enough turkey. We all over indulge on Thanksgiving so I'd plan 2 lbs per person. If you have some left over - great for you - freeze it for easy dinners. But wouldn't it be horrible if you didn't have enough for everyone!
2) Have someone else do the mashed potatos and you will need at least 2 crockpots full. This will be something someone else can cook the day before and warm up in the crock pots - p.s. it takes longer than you think to rewarm it.
3) Have someone else do the stuffing. Doing a good stuffing from scratch is extremely time consuming with all the chopping and cooking of the liver etc (if you do that), getting the bread stale. Someone can do this in advance too. We found great recipes on foodnetwork.com.
4) Maybe the folks who don't eat turkey can bring the ham. Seems fair to me.
5) I'd not do appetizers at all, frankly. You are going to have so much dinner to eat, don't let your guests fill up on muchies. Make sure they are hungry for dinner! :) Instead, have those people use their time preparing some of the actual meal, like a veggie. Some veggies we have are green beans (my aunt does something to them, I don't know what, but it's yummy), sweet potatos (wedges, not mashed), good 'ol corn, carrots.
6) I'd also skip the salad - but this is just my family preference. With all the veggies we have for sides, salad is usually not eaten.
7) Everyone should bring their favorite drink, pop or whatever. It should be BYOB or wine. You can provide coffee. Make sure you have enough glasses for everyone - we started using plastic cups a long time ago and we put our names on them because we got tired of wondering if a cup was ours or not. You may want to have a marker, just sayin' :)
8) Don't forget all your plates, silverware and napkins. Do you have enough "real" plates for everyone? Do you even want to use real? That will be a lot of dishes (probably for you). Again, we started using plastic plates a long time ago b/c we got tired of spending our holiday hunched over the sink - there are just too many of us. You'll need enough forks, spoons and knives not only for dinner but also dessert. You may want separate festive plates/napkins for dessert.
9) Don't overdo on dessert. Everyone will be so stuffed! We like to do a couple pies (you'll need a few), and a couple sweeter things like cookies. We also all sort of pick up a box of chocolates to pass.
10) How are you going to seat all those people? In our family, each household has a folding table with 8 chairs that we all borrow and pass around to whoever is hosting. We like to make sure everyone has a seat at a table and not eating on the floor or in the living room or something. You may want decorative disposable table covers for those.
11) Entertainment. What happens after you eat? We like to visit (of course) and play games. Most of the guys watch football. Also, everyone brings their holiday shopping mailers and we browse through those planning strategy :)
Sending you all my best! Good luck and remember to rally the troops! You need to enjoy your holiday too. Your guests should be happy to help considering all that goes into hosting!