Emulator

Attempted to bisect the issue, assumed 4.0-5025 however it's a dead-end with every single build producing Unknown Opcode. Looked into the Wiki history where compatibility and it looked to be 3.5 however those builds never never reached 5025.

Disabled Dual-core, no longer get Unknown Opcode. Most builds sit at a black screen with no audio after that. Some older builds (e.g 4.0.2-4572) produce Opcode errors regardless although after the errors they render a gray screen with a couple of white pixels at the bottom so possibly trying to render a frame and dying?

Not clear what you mean by the revs not going that high, we were at r5384 with the release of 2.0 (note these were from before the new numbering that came with the transition to GIT). That test report is really old. I think it's referring to the 2.x era r5025 release.

Here are the 2 closest revisions to that from our archive...
https://dl.dolphin-emu.org/oldbuilds/Dolphin-win-x64-r5021.7z
https://dl.dolphin-emu.org/oldbuilds/Dolphin-win-x64-r5130.7z

This issue started from this commit: https://github.com/dolphin-emu/dolphin/commit/05feabdb49cf4547cd016855a778ff3b2de5e40e

I bought a copy off Amazon for 6 bucks to attempt to help solve it, came out with:

--File Generated by CleanRip v2.0.0--

Filename: RM7E4F
Internal Name: Monster Lab
MD5: 00CC8421583AC459E6CEEC8B5C9A4ACF
SHA-1: 1E39B90C80124C8704ABAD2D25F99B4490929012
CRC32: 0B48EED5
Version: 1.00
Verified: No
Seconds: 1617

(note: it looks like the OP has PAL version, shouldn't be many differences tho)
it's using Bink Video (.bik), but I don't know if that's the issue, I was checking logs and OSReport prints out NAND_ERROR_NOEXISTS, so I dunno if it's trying to find/open a file and can't or something else, but that's all the further I've dove

Fixed with https://github.com/dolphin-emu/dolphin/pull/8039