Welcome

hmm, lots of questions Marc. And yes I know you from MKDD, I watch progress on all the ladders very keenly, so I know about everyone who tries MKSC

To answer sequentially...
1) Um... not sure, since the site doesn't operate on SC times, just non-SC. Someone who specialises in SC times like Vincent Zee could help you better there, I'm sure.
2) Fastest kart is Bowser. Toad makes for a good beginner, but unless your name is Kiyotaka Umeki or Shota Hirano, Bowser is best. The 'speed' rating actually should mean "acceleration", with 'weight' meaning "top speed"... its confusing.
3) Hm, same as any other game really... Practice your technique on the first, most basic track (Peach Circuit) and just work your way through acclimatising (since 40 tracks is a lot to learn). For drifting and blue boosting, Mario Circuit has a nice wide hairpin to learn, with a straight so you can practice 'carrying' blue boosts onto later places.
4) I don't understand exactly what you mean here; do you mean, why all the time totals of players are so spread out? It's just generally because the high number of tracks, varied SCs and techs and generally the variance in effort from some players makes the gaps quite wide.
5) We had a youtube account for vids on nearly every track, but it got unstickied

not sure where it is, though Pierre, Luca, Fried, Cole, Flemmer etc. all have top videos on their accounts.
6) Proof... well you wouldn't have to worry about proof until you maybe break top 20 (or set a top 10 or 5 time), just a camera recording of your replay should do. Mine are quite blurry but are still accepted, so as long as quality isn't atrocious you should be fine.
7) Of course! Enzio is a very good player, but I've seen your skills at MKDD and I think you could do just as well if you put the effort in. Thats the crux really: putting time into every track, not just 30 or 35, can be a key difference in this game.
Hm, someone else could probably answer better lol.