I'm not entirely sure what you did wrong, but here's what I did to get Isabelle. I've laid out every step nice and easy. I assume you have the tools to encrypt and decrypt your romfs.bin file and I'll assume you have a hex editor as well. This may require save editing, so I included a mini-guide at the end. Googling can help you find hex editor or save editor or whatever if you need them.
Here we go!
1. Delete your old romfs FOLDER and run the romfs decrypting program to get a fresh FOLDER from your clean romfs.bin file. This is to make sure you're working with clean files.
2. Inside the romfs folder, go to npc/special/model and make a copy of sza.bcres
3. Paste the copy of sza.bcres somewhere safe like in a new folder on the desktop or someplace you won't lose it. ALWAYS MAKE A BACKUP COPY OF ANY FILE YOU INTEND TO EDIT OR REPLACE!! I CAN'T STRESS THIS ENOUGH!!
Anyway
4. Go back into the romfs folder and go to npc/normal/model and find hrs01.bcres make a copy and put it in the safe place you made earlier. Yes, this is not cheri's file, we aren't replacing her, and I'll explain why in a few steps.
5. Copy sza.bcres and paste it into npc/normal/model
6. Delete the hrs01.bcres from the npc/normal/model folder. The copy you made earlier should still be in the safe place in case you want to change it back.
7. Inside the npc/normal/model folder, rename sza.bcres to hrs01.bcres
8. Now to Script/str and find the STR_NNpc_name.umsbt file. Make a copy to your safe folder then open the original (in the script/str folder) with a hex editor such as hxd.
9. Scroll down to the name Victoria. (It may look like V.i.c.t.o.r.i.a or be really spaced out, that's ok)
10. This part is important. Highlight each letter individually and replace them one by one with the letters in Isabelle's name. I chose to replace Victoria because she has the exact same number of letters in Her name and the right personality to fit Isabelle's natural bubblyness.
Make sure you don't accidentally overwrite or delete any of those . or spaces. The game needs them there. You can probably do this in a basic text editor like notepad, but its safer in hxd where you can clearly see the letters and the hex values between them.
11. After you safe that file, you're done. Encrypt your romfs folder to make a new romfs.bin file. Save this file as 00086300.romfs (assuming you're using the US version, otherwise rename it as whatever the last 8 digits of your gameid. Be sure you use the right name. Chances are, braindump extracted a folder with a 16digit number as the name, the last 8 are the ones you need.) Drop that file in the Hans folder on your sdcard and you're ready to go.
To see your changes, just use marks save editor to edit your garden.dat file and change one of your villagers to be "Victoria" (his editor doesn't know its really Isabelle) use svdt to copy your garden.dat file from your game to your sd card. When using svdt, it will ask about preserving the data when reinstating saves, select yes whenever this is asked so that you don't corrupt your save file. Repeat this process in reverse to reinsert your edited save file.
Always keep an unedited copy! That safe folder you made earlier is a great place to keep unedited files in case you need them back and you don't want to remake the folder over and over again.
I can confirm this works as I have replaced many villagers many times without trouble.
Also, please do not double or triple post. I understand that you are frustrated and want answers, but it is rude to post multiple times in a row. Please understand that not everyone on this forum will be able to help you since the active rom editing community for this game is very small and its tough for us to get together to share our findings and to help newcomers with everything we have going on. Thank you for taking the time to read through this, I hope that this post helps you out.
