Whirli: yes. I know, shocking.
To answer the modding question:
You can search for a date for this one, but the best I can find is roughly towards 30th December 2003, though no actual release. The first release is 31st Jan 2004.
The definition of this mod is a very soft mod, for it is LillBenny's At the Airport, which added a skybox and modified a few textures.
The next soft-mod is probably Fun With Floors, which added new friction floors to init.cs (provided with textures, interiors and a level). It released (?) on 3rd April 2004.
This actually links to Custom Floors
which is the same thing just without the level (with the level it became FWF).
Other soft-mod was Cactus Math by Solo Hans, but that was just a skybox.
The next "MOD" was from LillBenny as well. This was about 5th July 2004. It changed marble skin, bumpers skins, button skin, lots of textures and the skybox. It didn't have any other script modding.
So the first mod would definitely be Fun With Floors.
There used to be a MB Mod Installer which is an installer and archive creator for MB Mods and levels in .mbzip format...
BUT WAIT. THERE'S MORE.
You're welcome.
Links should work too.
The first real mod that wasn't just one script, but the full thing, would be Marble Blast Platinum. No other mods exist to the game beforehand.
"matan, now i get what you meant a few years back when you said that "the level in mbg is beyond me" after the last rampage i noticed things were insane, and now i truly feel that too" - Dushine, 2015.