I didn't try to mod the game but I think this way

Overall, depends
I feel like with coding knowledge, it’s pretty easy to make something like Garcello or one of those simpler mods
Mods like Hit Single, Weekly, and Mario’s Madness were most likely much harder to make

The hardest ones are the Fatal Error and NMI vs Tails mods, as they can do shit with your entire pc

Also without Fading, you can pretty much animate your own cutscenes so you can just replace Week 7 or Week 3 Pico Mix

Oh yeah I forgor about the playing with your pc things
And also yeah, the cutscenes part is prolly true