This is written for future me & for future other-people who want to install something like vivecraft that create new versions of minecraft. (I don't know of any other loader, but I'm sure something must exist.)
Problem #1: Library's
Library's may not download correctly. The simplest fix is to copy the whole library's folder from your .minecraft and host it on the webserver. (You can figure out which ones you actually need, or just copy everything.)
Note:
Remember to put the library's folder into the test webserver directory for testing.
Problem #2: Inheritance
It turns out that minecraft versions can inherit. Well, we can't have that, so you will need to go down through the inheritance tree merging the library's into the top version. (And don't forget to copy the relevant library's!)
Example:
For vivecraft 1.12.2 (Forge) you need both the library's in vivecraft-1.12.2-jrbudda-9-r6-forge.json and in 1.12.2.json merged together. I put the base one at the bottom but it probably doesn't matter. Just make sure it is valid json.
Problem #3: Signing
Since we are horribly butchering the jar, we need to disable hash checks. Ensure that the following is in the modpack properties under launch:
-Dfml.ignoreInvalidMinecraftCertificates=true
-Dfml.ignorePatchDiscrepancies=true
And with that everything should run.
