First, congrats on kids after infertility! Okay. There's one thing you said that I completely disagree with: She IS old enough to reason with. I have a daughter that's a year older than yours (also the oldest) who has the exact same problem...STILL. My solution to my own daughter's picky eating problems was to let her eat whatever she wanted for breakfast and lunch, but at dinner time, she has to eat what we're eating. If she doesn't eat anything, she doesn't get anything until morning. Sometimes, she goes to bed without having had anything since lunch. But a lot of the time, she'll at least eat a few bites of whatever we're having for dinner. I also try to make something with dinner that I know she'll eat. Like peas. My daughter loves peas. She hates mashed potatoes, any kind of meat, or anything that has been mixed together, but at least she'll eat the peas with dinner. There are a few other things like that she'll eat. But mostly, she just likes the snack food like fruit snacks, granola bars, graham crackers, milk, etc.
Also, I wouldn't give her the instant breakfast. Even if she's not eating much else. At that age, even if it seems like she's not eating much, she probably is getting all that she needs with regular milk, and whatever food she does manage to get down. I've been told that by my daughter's doctor, as well as many other moms who have had the same experience. The instant breakfast in the milk is probably hindering your efforts to get her to eat "real food." She'll feel more full and satisfied by it, and will have even less interest in eating anything sustainable.
Another thing, try to get your husband to understand that giving her 10 different options until she actually does eat something is not helping. I don't think it's "off base" or unreasonable to expect your daughter to eat what you make instead of catering to her. Maybe compromise on some meals by giving her what she wants to eat, but on others, let her know that she has to eat what is provided. Maybe let her choose her lunch or snack, but let her know she has no choice when it comes to dinner (unless you want to ask her opinion on what to make for dinner. This might also help with her eating. If she feels involved in the process of making the meals, she may feel more inclined to eat what she helped make.)
Another thing that helped my daughter was a sticker chart (which we started at 2 1/2 - a little younger than your daughter). The chart was for many things, which included eating. She knew that when she filled all the spots on the chart with stickers, she would be able to choose a special toy or movie or whatever from the store. It helped for a while, but when she completed the chart, she reverted back (though not as far) to the disinterest in eating. I think we probably should have started a new one after that.
Well, I hope my response helped. My daughter's problem with eating stuff she doesn't want or like isn't completely solved, but she definitely eats a TON more than she used to. And a lot more variety now. Just know that your daughter probably understands a lot more than you would think she does. And if her behavior continues, it may start rubbing off on her younger brother. He'll see that if he begs and pleads and bargains, he'll get whatever he wants to eat too. Then you'll have TWO children with the same problem! Hopefully it won't come to that. Good luck!