So with Celebrity, you can pretty much order what you want. Also it sounds like you are looking at a Solstice Class ship so my answers are geared that way.
If we go in and ask for a salad, a soup, an appetizer and a entree plus a half size of pasta (thinking of Tuscan here) plus a side or two, no one would blink an eye at the order. If it were me placing the order, I may need to be rolled out of the restaurant, but Celebrity would be fine with it.
The $89 option in Murano includes the wine paring for the menu. The “regular” menu you can choose as you go and pair wine or drink whatever you may choose.
The most “restricted” menu we’ve encountered is Le Petit Chef - they have two menu options and one follows the storyline of the video, other does not. It comes with four courses - you choose an option from one of the two presented menus.