The extended offline campaign, featuring the main characters sibling, has been unplayable on all of the modern platforms for which the game has been released. Until now the only way to play this has been if you had a Japanese Wii and the original media (discs and 16gb thumb drive).
This workaround uses emulation to get the original Wii version running on a (modest, yet capable) PC without the need for a Japanese Wii or the thumb drive. You will, however, need either the original game discs or an image of the discs. Procurement of the images of these discs WILL NOT be discussed here, nor will the images be made available here.
What you need:
You need to make .ISO rips of your game discs. There are many free programs available for this sort of thing.
You will need about 8-10gb of space on your PC hard drive as Dolphin Emulator installs the game channel on your HDD as a sort of virtual version of the 16gb thumb drive. Once it is installed you may delete the .ISO files to save space as they are no longer necessary.
Thanks to our friends over at Dolphin Emulator we have a working Gamecube/Wii emulator to run on our PC's. The first step to getting this going is to go to https://wiki.dolphin-emu.org/index.php?title=Dragon_Quest_X:_Mezameshi_Itsutsu_no_Shuzoku_Online and scroll down to the Configuration section. Above it you will see "Fixed in 5.0-5291." Click the numbers to go to the download page and then download Dolphin Emulator version 5.0-5291.
*The current version of Dolphin Emulator WILL NOT run DQX. The only working version I know of as of the writing of this is 5.0-5921*
Once you have unzipped Dolphin run the .exe file inside to start the emulator. Choose TOOLS>Perform Online System Update>Japan. This will connect to the Wii servers and set up a Japanese System Menu in Dolphin. You will need an internet connection of course.
Next press the Graphics button on the main menu and choose the HACKS tab. Check the box that says "Skip EFB Access from CPU." The game will not boot correctly if this is unchecked.
Next go to FILE>OPEN and select your disc 1 .ISO file that you ripped from your game disc. It will load the game installer and have three choices. Choose the top to install. It will then do a countdown from 100%. Once it reaches zero it will ask for disc 2. Tab over to the Dolphin main menu and choose FILE>Change Disc then select your disc 2 .ISO file. It will do another 100% countdown and then say that the game has been installed. It will then restart to the Wii main menu. Choose the DQX channel to begin gameplay.
In order to play again later you will open Dolphin and go to TOOLS>Load Wii System Menu X.Xj. this will load the Wii menu and then you can select the game to begin.
I was able to run through the prologue and begin the extended offline portion without any trouble once it was set up this way. I set up an old PS2 controller so it felt a little more like my time on the PS4 version. The graphics can be tweaked a bit to make them a little nicer too, if your system is capable of that sort of thing.
Anyway. Cheers folks and enjoy.