That was the condition of the OP – this can’t be buffet-style, it has to be plated. 
bolding added
Antipasto, quiche and a whole grain roll
This.
50 people, whose needs you have no idea of, and who are unlikely to have the same ones? Whatever you put on that plate – even if it is plain white rice, most certainly if it’s pasta – the chances are really good that there’s going to be somebody who can’t eat it.
And you’re going to have to be really careful about both that buffet, and everything that comes in/has come in contact with the food, if somebody’s full scale keeping kosher.
People shouldn’t be offended that others are eating food they can’t eat, no. But they also shouldn’t have to have a plate put right in front of them filled with food that they can’t eat, or even with food that might make them actively nauseated, whether for physical or philosophical reasons. There needs to at least be an option for ‘no, don’t fill a plate for me’; preferably combined with an option for ‘I’ll eat what I brought with me.’
The issue isn’t only that nobody’s going to literally starve. Some people may be too hungry to concentrate on whatever they’re supposed to be doing, or learning, at the rest of this “business meeting or seminar”. How long one needs to go without food for this to become a problem varies a lot with the person.
The whole setup is just plain a bad idea. I presume that FairyChatMom isn’t the one who decided to do it that way, but has just been stuck with it; but is it possible to at least get in an option, decidable in advance, for ‘don’t give me a plate, I’ll bring my own’?
I only answered that way because it was part of the condition in the OP. I actually pride myself in being able to feed my friends, and I have made food that was safe for, and enjoyed by:
vegetarians
vegans
celiacs
nut allergies
kosher-style (including kosher-for-passover)
lactose intolerant
a guy with a deadly nightshade allergy
a woman with major sensitivities to chicken (including eggs) and corn (including corn syrup and corn starch)
paleo
allium sensitivity
melon allergy
diabetics
picky eaters who don’t like some of fish, peppers, cheese, or capsaicin
often feeding people in several of these categories at the same meal. I always invite people to bring their own food in case I don’t have things they like, but the only time I gave up and said, “please bring something you can eat” was when a vegan friend was celebrating Passover, and I realized the only foods I could easily provide for her were plain matzo and charoset, and that wasn’t going to cut it for an actual meal.
But “I’ll be really offended if there’s a wrapped food item on my plate that I can easily remove” is a new one to me.
I mean, yes, of course it is better to have a buffet, with well-separated foods. And that’s what I do for parties in my home. But sometimes they hand you a box or something, and (as a picky eater) I have always just picked through it to find the items I can eat, and given away or discarded the rest.
fwiw, in my experience, at something like a hotel banquet, the organizer will be charged more for a buffet than for a plated meal, because (1) people eat more and (2) there is MORE waste, because they don’t want to run out of anything, and don’t know how much the first people in the line will take. So the buffet doesn’t prevent the food from being wasted, it just makes it less obvious to you.
(even for home parties, there is wasted food. Because the vegan will bring an enormous tray of some vegan dish that isn’t super-popular, and that my family doesn’t like. So we throw away those leftovers after she leaves.)
Understood.
The plated food is going to be still sealed up in its packaging? I didn’t see that in the OP. Generally plated food isn’t wrapped.
It was in my suggestion – wrapped half sandwiches, to facilitate trading across the table.
Ah. Yes, that might work. You could even source some of them as kosher; as long as they only touched their original wrapping, they’d stay kosher.
I was directly addressing the OP in my meal plan, but I would never set up a dinner that way.
I’ve frequently had to cook for big numbers at medieval banquets, even plated ones rather than semi-buffet style (4-6 people sharing a selection). It’s not professional catering, but it is that scale.
People should always have the opportunity to communicate dietary restrictions before the meal, and I’d publish my planned menu, based around that, before the event. There’d always be choices at each remove (course), and no-one need ever bring their own food (except the strict kosher keepers). Now, people being people, I’ve often enough had people turn up and then go “but I’m allergic to X” or “I don’t eat Y”…and we try and accommodate them off the available foods anyway.
Except people with the severest tree nut allergies. They shouldn’t even be in the same hall as medieval food, because there is going to be nut somewhere.
In similar situations, I have been served the ubiquitous grilled chicken, rice pilaf, roll, simple vegetable, and garden salad. Iced tea or lemonade to drink, slice of cake or pie for dessert. I don’t know if accommodations were made for dietary restrictions.
The fact that it both has to be plated, and you can’t offer choices makes this really difficult. If choices were allowed I would take a cue from airline catering, since they literally deal with this every day, and offer a choice of something like chicken or pasta. The pasta dish would be something vegetarian, like a cheese ravioli or vegetarian lasagna. That wouldn’t accommodate strict vegans, but it would still accommodate vegetarians, and pretty much any religious requirements. The chicken dish would be something like what Tarataratara described: grilled chicken, rice pilaf, some sort of vegetable. Both options would come with a green salad, roll, and something like a brownie for dessert.
But, since the OP disallows offering choices, I guess I’d just do the vegetarian pasta option, since that seems like the one that would accommodate dietary requirements (but not all). Yeah, there might be some people annoyed by not having meat, but I would guess most would eat it just because it’s a free meal.
Straying a bit off topic, but I have noticed that on most long haul international flights I’ve taken in recent years, one of the two “regular” meal options you can choose if you didn’t pre-order a special meal is always vegetarian, usually something like the pasta dish I described above. I assume they do that to accommodate any vegetarians and people who keep kosher/halal who neglected to request a special meal.
Well, I hope you didn’t give him a tip. 
On top of the gratuity? No. Plus one is at the mercy of your hosts best to go with the flow.
This situation I related happened in the late 90s so I don’t recall many details. I suppose there were some in the group who may have just picked at a dinner roll because of the food that was offered. At the time, I didn’t really think about it, but when a conversation with my daughter brought the event back to mind, it occurred to me that the lunch choice was pretty risky, considering how many people I’ve encountered who won’t eat fish.
It also reminded me of a story the late comic John Pinette told about when he was making a movie in France. The catered lunch was rabbit. Maybe it was a local favorite, but I think it’s at least as risky a choice as salmon.
To answer my own question, I think a salad and sandwich assortment is probably the most benign choice, provided the group isn’t full of celiac patients. Fortunately, I’ll never have to plan such a meal.
The correct answer, of course, is don’t do that. The problem gets a whole lot easier when the diners have a choice of at least two meals. Even then, you’ll never be able to feed everyone (get a group large enough, and you’ll eventually have a celiac vegan with allergies), but you’ll have a much easier time satisfying most.
That said, working with it as an arbitrary puzzle (i.e., not fighting the hypothetical), for starters it should probably be vegetarian. That’s a fairly common dietary restriction, and it also makes it a lot easier to meet most of the religious restrictions. But it should also be something at least familiar to the carnivores: Any sort of “fake meat” is going to be unpopular with a lot of folks. Pasta dishes are probably the safest bet: Spaghetti with tomato sauce would meet a lot of requirements, but lasagna, while probably involving cheese and thus being non-vegan, is easier to make on an industrial scale.
Of course, any sort of pasta runs into the gluten problem, and gluten-free pasta, while perhaps a necessary evil for celiac sufferers, still sucks. Something Mexican, maybe, so you can make the carbs corn-based instead of wheat? Though everything being bean instead of meat would call attention to the fact that it’s vegetarian, in a way that pasta with red sauce wouldn’t.
Something Middle Eastern based around falafel should be a good option, since it’s vegetarian, but most meat-eaters who try falafel like it. But now there’s the question of how picky your diners are: A lot of folks probably aren’t going to want to try it.