I think you have good rules, too. I hadn't thought about letting the kids pick a dinner--I should try that. We serve one dinner and I only offer another option if I accidentally make the food too spicy.
We usually have bread and butter or a veggie or fruit that I know the boys like. I ask them to try a bite but don't push food beyond that.
I think the key is endless repetition. The rule is the rule is the rule. Both my boys have had nights when they did not eat dinner and unless your ped is concerned about underweight issues, that probably is OK.
I have no idea how to contain the dawdling. I eat too fast because my parents ate fast and so I like that the boys can linger, but an hour for a piece of chicken and some broccoli seems extreme!
When one of the boys gets up to play, the plate is cleared and dinner is over. DS2 is very serious about his food and stays at the table--DS1 needs reminders. We are trying to institute "may i be excused?" to clarify that the child in question is done eating.
I have to tell you, most of the kids I know who only eat a few things have parents who are willing to offer alternatives at dinner. I am sure there are exceptions to this, but I definitely see a pattern. I was not willing to be a short-order cook. I do offer a couple of options for breakfast and lunch, but I do easy things like sandwiches, oatmeal, toast, etc.
With a few exceptions, we do not give dessert as a reward. If we have dessert (sometimes fruit, sometimes a treat), the boys can have it if they eat even a little dinner. But we do not always have dessert and if they eat a cookie or whatever and are still hungry, we go back to the chicken and veggies, not another cookie. We talk about foods that are fun and foods that help you grow big and strong.
Hang in there--late-stage pregnancy is tough and you need to do what works for you right now.
For you to be here now, trillions of drifting atoms had somehow to assemble in an intricate and intriguingly obliging manner to create you. It's an arrangement so specialized and particular that it has never been tried before and will only exist this once.
--Bill Bryson, "A Short History of Nearly Everything"