Also sorry for the unneccessary bolding and double post. was using asterisks for wildcards.

You can use the LoadBundle script. You just have to point it to wherever (ex. C:\users*your username*\downloads***.vrca) you saved the VRCA file. Also it looks like you didn't rename the original prefab when you were working on it in unity. Drag the avatar prefab (after fixing/removing all missing scripts on the original prefab to make it saveable) into the hierarchy. Rename it from the prefab-id**********.....whatever the name of the prefab data is for the avatar. After you hit run with the loadbundle pointing out on the right path to the VRCA file, it'll create a prefab-id....Then use the pumkin tool to copy the data to the one you renamed. Copy/drag the new (working copy) it into the assets folder and then you can delete the other items in the hierarchy. Drag the newly made (working) copy into the hierarchy and upload!

Tl;dr Yes you can use the LoadBundle Script, you just cant point it to a VRCA file out on the internet. Point it to a local copy on a directory and it'll work.