I've made a tool for the .ntsp files, check here in Sonic Frontiers folder (also read README for usage instructions): https://github.com/blueskythlikesclouds/SkythTools
I'll share format specification & source code later.

Yeah, it is only available for registered developers. However, they send it to basically anyone if you fill their form. The key I sent was in decimal, it appears it was put as hexadecimal in that wiki page. Perhaps that's the problem. I tried the tool you mentioned with the hexadecimal version of th...

Found the key for HCA and USM files: 53346933792338754

I was able to play the movies in the official CRI SDK tools by providing this decryption key. I'm not sure if there is any community made tools for it, though.

I've made an extractor for the new .farc files which is also compatible with the older iterations. You can find it in my repository:
https://github.com/blueskythlikesclouds/MikuMikuLibrary

EDIT: Updated with the latest version of the program.