I can't say much about staying in his seat -- my 3yr old is still strapped into her high chair. (If she starts acting out, though, I often take the view that she must not be hungry and take the food away.)
With regard to trying new foods, I think some fussiness at this age is normal and they outgrow it, but I have followed Ellyn Satter's book "Secrets to Feeding a Healthy Family." I highly recommend it!
The basic idea is that you prepare a meal and eat with the child. (This is not achievable in our household at every meal, either, but we do our best. Sometimes I eat with DD and just sit with DH when he comes home later.)
The parents control the timing of meals (3 meals a day and one or two planned snacks -- nothing but water at other times) and the food served. The child controls whether, and how much, he eats of any food. Make sure that there are foods like bread and butter so that a new food doesn't create too much pressure. They might not try things the first time, but just getting used to seeing it helps them recognize it the next time.
I cooked a white bean, cabbage and chorizo dish last night and didn't really think DD would eat it. I served hers on pasta, which she loves. Not only did she eat all the pasta therefore at least tasted all the new flavors she also ate all the chorizo, which was surprising to me.
The book says not to discuss eating at all, but I can't resist at least prompting her to try things, especially telling her it is similar to a food she likes. But if she doesn't eat, I say, "Well, you must not feel hungry." If she eats a lot, I say, "Well, looks like you were hungry."
There are no negotiations or substitutions. Both sets of grandparents bristle a bit when I say, "No, that's what's for lunch/dinner" and don't let them fix an alternative -- but they are also really pleased to see her dig into an unfamiliar food and eat it with enthusiasm. She wouldn't do this if I always caved in and gave her macaroni and cheese. Just serving things that you know the child will eat just narrows the circle of acceptable foods because ultimately they get sick of their favorites as well.
I don't serve dessert every day but when I do it is not dependent on whether she has eaten anything else. Dinner isn't something you gag down to get a treat -- you eat it because it's good for you.
A lot of this goes against the grain for me I want her to eat what I serve, and clean her plate. But she only weighs about 15.5kg, so having a small piece of cake at playgroup can make a difference in her appetite, and the last thing I want is for her to learn to eat when she doesn't feel hungry. Your post talks about "standing your ground" and being "firm" but I have learned to see it as being relaxed I make dinner and put it in front of her and she takes it from there. When she says, "Don't want [X]!" I say, "You don't have to eat it, but it's what's for dinner." The record was when she ate nothing but ketchup and an orange for dinner. I was disappointed putting her to bed, but she was fine!
It also really helps to have her help prepare the food. She gets really excited about trying it. She can help measure things into mixing bowls, stir, grate cheese. She can also stand on a chair with me between her and the stove if I'm cooking on the stovetop.
We didn't have a big problem starting out but I think she does really well, eating a variety of foods and trying new things.
Sorry this is such an essay! good luck