Hi EJW1,
Just for your information, Claude and others have suggested that for this kind of bracket, the best solution is to use the tuplet function (and choose not to show the ratio, of course). No need to do it in the end, and if you want to change the looks of your score, the brackets will stick with the music correctly.
As for brackets, if you choose the right option in Engrave options (cmd-shift-E) > Brackets and Braces — Small ensemble, Dorico will follow the groups that you create with your players. If you put one group/player, they won’t show brackets (that’s how I use it for opera). Fratveno could have created special players for his prefatory staves, with a group for each of them. Just guessing here 