I think it's important to separate issues from non-issues.
First off, there is no rule or assumption that should be made about "only children". I prefer the term "single child" because "only child" implies that there is something missing. There isn't. Families come in so many configurations. There is no specific set of behaviors applicable to single children. Some are independent, some are not, some are selfish, some are not. You could say the same about middle kids and 7th kids… So let's set the labels and assumptions aside.
Next, there's a big difference between choosing what you want for dinner and rearranging the furniture in the room. Of course she should be able to move the furniture around in her room without asking. Other than scratching up the floor or having a young child move a bed close to the window (falling hazard), who cares? Now, a self-serve kitchen? That's another issue. It might be more convenient, and if there is a natural process for choosing a balanced diet, fine…at least on the surface. But any group (family, roommates, classroom, work environment…) requires some sacrifice and cooperation. If your daughter didn't learn to be considerate and cooperative, that's a problem that will haunt her for years. When the dynamics change and the make-up of the family changes, then some cooperation has to be instilled.
So while I don't think you should police her every bite of food, I have to ask you why she is hungry after a sit-down family dinner? What message is it sending about when someone should eat, how late someone should eat, etc.??
But I also think there are different rules for different ages. Toddlers nap, grade school kids take soccer, tweens go to the mall alone, 17 year olds drive….and so on.
Teaching your daughter that she has to be adaptable when the family situation changes is a life skill you need to give her. She will, sooner than you think, go off to college and have one or more roommates, as well as hall mates. She will be sharing a room, a communal bathroom, going to a dining hall, and more. She needs to learn to be accommodating. She needs to be sensitive to others' needs, routines, cultures, maturity levels, and schedules. "I do what I want" is not a workable life philosophy.
So you sit her down. You explain that she's mature enough to make some adaptations. She's a role model. She can defer some of her immediate wants (which are not "needs") and she can do certain things when the little ones aren't looking. She can be patient. She can give up her wants for the greater good.
The little ones can learn that she has privileges because she has more responsibilities too. She does laundry and she sets the table and she babysits them. They do nothing - because they are little. So the rules are different (and teen rules are different from adult rules), and when they grow up, they can do more in increments.
So, your daughter can learn to eat dinner with the family and not just push away what you have cooked because she "feels like" a PBJ later on. What are you teaching her about staying at someone else's house (to reject their meals and help herself??) or helping to prep a family meal and enjoy that?
Does she help with the meals? Does she clean up the group dishes as well as her separate PBJ dishes and utensils? Or does she selfishly leave everything for you because you don't want to give her structure and rules? Big difference. Is she helping to teach the 4 year old that the meals don't miraculously appear, but that the table needs to be set and cleaned up, the leftovers need to be put away, etc.?
There's also a huge difference between asking "before you do ANYTHING" (as you say), and having some decency and consideration for others. If you've given her too much free rein, you've handicapped her for life going forward. If she cannot work in a group, she's absolutely screwed in the workplace and in a marriage and in college. So sit down and figure out some adjustments for each person - and get family therapy if you need to - so you have a workable solution.