How about boxed kosher chocolates? Dark, not milk?
The last time I went to someone’s house for dinner who kept kosher, I brought some lovely flowers.
Roasted winter vegetables is the answer. This can be rich and dense and very Thanksgiving-y, no gluten, no dairy, no animal products.
By “winter vegetables” I mean squash, sweet potatoes, carrots, parsnips, celery root, turnips, onions, beets, cauliflower, or whatever you like but they should be dense and deeply colored. Toss with olive oil and whatever spices you like. You know, garlic. And salt. Or go more sweetish. And then you roast until 98% done, and finish off in the oven of your host.
Or get some delicata squash. Cut in half, drizzle a little mild oil (you should use butter, but oil is fine). Sprinkle a little cinnamon, cloves, and some brown sugar. Roast. When you get to the place, put them back in the oven for 15 minutes. Now you’ve got dairy-free vegan gluten free pumpkin pie. I say delicata because you can eat the skin and after long roasting it gives a little bit of crispy contrast to the squash meat. Top with toasted pecans.
The key to dairy/gluten/meatless/whateverless cooking is to not start with "I want food X, give me the Y-less version of X. It’s gonna suck. Gluten-free dairy-free pizza is a ladleful of marinara sauce on a plate. But if you want a pizza-ish or pasta-ish experience, don’t get crappy rice noodles or a pre-made frozen crust and soy cheese. That’s gonna be horrible. Instead, make polenta, and top with whatever italian-ish sauces and toppings you like. Polenta isn’t a crap substitute for wheat, it’s an honest food.
So trying to make “pumpkin pie” with gluten-free dairy-free pie crust is gonna be terrible. But roasted sweetened spiced sweet potatoes or squash gives you the essence of pumpkin pie but without any fake ingredients.
That’s because “Kosher salt” is a bit of a misnomer. It’s properly “koshering salt” because it’s used to draw out blood, but it’s not necessarily kosher, although it probably is, most salts are - (I can’t think why they wouldn’t be, maybe a higher-than-background brine shrimp content?)
I sped read the thread, but cranberry sauce made with real cranberries, boiled with sugar in water, tastes much, much better than the stuff that comes in a can.
What about succotash? It definitely fits with Thanksgiving. Just pick a recipe that doesn’t use butter, like this one:
Like I said, I’m not really an expert on all things kosher. It has a Star of David on the box and says it’s kosher certified for preparing kosher meals, I just figured it was kosher salt.
I think it better to get something from a kosher kitchen, like a deli or grocery store and bring that. That way, there is no doubt that the food itself not only is kosher, but it was prepared in a kosher kitchen, so can be eaten and enjoyed. There’s plenty that you will be able to eat too, from some of the other poster’s suggestions. 
Edit: Some are forgetting that not only does the dish have to be kosher, but the method of prepartion, including the entire kitchen it was prepared in have to be kosher for the very observant.
I had to re-read your post, and after some thought and consideration, you could do this:
a) Make some Grilled portobello mushrooms for your “meat”, and grilled/steamed asparagus for your veggie dish.
or
b) feign illness and bag the dinner.
I like the idea of some Kosher wine or chocolate, or other snackage that is Kosher. Something boxed and/or labelled so they’ll know it’s safe.
I’m not sure I’d even bother trying to COOK something and take it over to someone’s Kosher kitchen - honestly, they probably wouldn’t want to eat any of it anyway since they didn’t watch you making it.
Whole, uncut fruit is usually acceptable too.
If you Google - gluten free vegan recipes - you will find hundreds of things you can make. Just pick something that’s also potato-free and you’re golden.
Since the host said you can bring food within the guidelines she gave you, it sounds like they are willing to eat something you make, so not the absolute strictest kosher, and at least you can have a main dish you know you can eat.
I eat vegan when I’m not at home, and the advice early in the thread to eat before you go and pack snacks is solid advice. I usually just assume there will be very little I can actually eat and prepare for it. If that ends up not being the case, then I have extra food I can take back home. I make an apple-bean salad that’s vinegar and oil based, is very filling for me if I need to have some extra as a meal filler, and everyone likes it so it’s a good dish to pass.
One of my favorite Thanksgiving dishes is wild rice “stuffing”. Saute diced shallots (or onions), celery, and mushrooms until soft. Mix together with cooked white and wild rice and then re-heat in the oven before serving. I like to cook the rice in turkey stock but it’s not necessary. You could add chopped almonds, pine nuts, or sauteed giblets.
Uncut fruit? I would have figured circumcised fruit more appropriate. Huh.
You got there before I could. L
If I were in your shoes, I’d just bring a box lunch.
Hopefully the cardboard isn’t made of pork byproducts.
Duh, she’s hinting that she’d like you to bring wine.
This is what I was going to suggest. Don’t bother bringing anything for the meal, bring a hostess gift for the mother like flowers or candy.
I’m slightly confused - has the OP shared his food restrictions with the hosts? If not, why not? They shared their dietary restrictions freely with him. I would feel mortified if I had a guest, and they couldn’t eat anything I cooked at a family dinner when I could have easily accomodated them in at least a few things (for example, not stuffing the turkey, making sweet potatoes without any gluten-containing ingredients, etc.)
before you leave eat an Atkins Cheese Burger
Thats 2 burger paties with your favorite cheese in between.
then commence with the drankin.
food allergies suck ass, esp with all the processed foods out there that are determined to use the most common allergens in every damn thing they make.
Another reason kosher salt is kosher is that it doesn’t have any additives such as iodine (or to be more accurate, iodized table salt ISN’T kosher because it DOES have iodine in it).