Hi! So this will be a long tutorial hope you understand it all
To scout your version exclusive trainers there's a video about it here https://www.youtube.com/watch?v=IE7ZkmfV8PY&t=496s
You don't need any hacks but make sure to follow the instructions displayed in the video for a successful scout. About the adding different characters that are not scoutable in your version.
Make sure you have a backup of the GARC someplace else than the one pk3ds opens in case things go wrong.
Use pk3ds (make sure you have the latest version), go to the royal battle/battle tree editor and pick battle tree by pressing no. You will see the pokemon you can add or delete. You can also edit the existing pokemon. By what I red I think you already know to do that. You will notice that on the top left side there's the trainer info and that's what you will need to change. You can change any trainer but for the pokemon ultra I recommend changing either Rada or Lillie (since they both easy to obtain). For Lillie you need to talk to her post game and let her know you want her in the battle tree, as for Rada she's there from the beginning. I for example changed Lillie because she had less pokemon to edit than Rada but you pick the best. So we're at the battle tree menu all you got to do to change the model is select a different Class. Really is that easy! Faba for example is aether branch chief. Changing Lillie class to the branch chief changes Lillie with Faba. But it's not over of course. If you only do this modification Faba will be called Lillie. So it will be shown in the game as aether branch chief lillie lol XD and you do not want that so the next step is to change the name.
Go out of the battle tree and go to game text. Do not change anything yet! First export the vanilla txt. by export .txt on the top. Now open the text that was created in the folder you placed it. Search for the trainer name you will replace (rada or lillie). In here you're searching for a line of text that only shows her name like "Lillie". For Lillie for example there's more than one line like that but I found that the Lillie under the 104 group is the one to change. You can open text editor, go edit and locate the word to see how many do appear.
So after finding the name of the trainer, go to the game text editor again. In Lillie case go on text file select 104, if you want to change another trainer you will have to find it yourself but if you're having problems I can help here. In the text file you selected (in my case 104) search for your trainer name. When you find it is very simple, click on it and change it to the name you want. Done!
The next step is changing Lillie introduction text to something to your character liking. Is the same process as changing the name but on another text file. I haven't work on that yet but I imagine it should be on game text too. See what lillie says at the battle tree and then try to find that on the vanilla .txt so is easier to locate.
And done! I think there's nothing else that needs changing from what I tested but let me know if you find problems. Ok! You're done with pk3ds now the next step is making it open on game using luma. Go to the pk3ds folder, backup, (pokemon game name), a. You will find there the location of the garc files. You changed the following files gametext(a032) and (a281, a282,a283,284). Go get the edited garc files at the folders. Just follow what the backup pk3ds tells you.
Ok once you got them all supposing you have luma go to the sd card. Go to the following folders or create if you don't have them. Luma-titles- folder with titleID (put here the titleid of your game, let me know if you can't find it online) - romfs - a - (then the rest is the path indicated depending on the file). In case of the game text is a (folder) - 0 (folder) - 3 (folder) - 2 (garc file).
Put your sd card on the 3ds. Hit select before turning on the 3ds. You're now on the luma3ds configuration menu. Go with the arrows to enable game patching and click A on it. A cross should appear letting you know is activated. Select start to save and the console will start. Now you test it! If you done everything right should work like I showed in the video but if not let me know to see if I can help. Hope I was helpful