boss going off track in canyon event
This can only happen with Wolf in the tier 2 boss canyon event, and it's both rare and scripted, always at the same point. Wolf isn't killed since you seem him later on in a tier 3 race and also the first two of the final four boss races.
tier 3 cars unlocking early
As soon as you defeat the final tier 2 boss, tier 3 cars are unlocked and will show up in any race after that, including a defend race that can randomly get triggered in the race where you defeat the tier 2 boss. If you're in a tier 2 with AI in tier 3 cars, the game will compensate, but tier 3 rubber banding is much more challenging than tier 1 or 2, regardless of how fast your car is. Using speed breaker (slows down time) usually defeats the rubber banding logic, while using nitrous tends to cause the AI to speed up as well.
Fastest car is the Cop Z06 (classified as tier 2 muscle, with almost nothing showing on performance bars), but can only be acquired with a car unlocker or perhaps hex editting a save. Unlike most cars in Carbon, it doesn't oversteer, similar to the CCX, but with much more power. The Cop Z06 can be used for career mode, but if you click on select car in the garage, it will disappear. The Cop Z06 doesn't drift well due to it's high grip, but it can score enough points to win the career drift events. Fastest tier 1 car is the 240SX (which may be a collector's edition only car?). Fastest tier 2 car is the Elise (R32 slightly better for canyons). Fastest tier 3 car is the regular Z06 for most tracks, and the Audi Lemans is a bit faster at a few tracks (like Condo Row).
PC online reward card - beat a moderator
This was never possible, because when EA created the PC database for Carbon accounts, they didn't include a field to note that an account belonged to a moderator.
It was running as of 1 or 2 weeks ago. Occasionally I'll set some weekly records on the leaderboards, trying to get friend requests, for possible online play, but for the last year or so, there is very little leaderboard activity, and almost all of that is players doing career mode events while logged in.
World versus Carbon versus Most Wanted handling
The cars in World don't handle the same as either game. Although the graphics, and perhaps the physics of World were based on Carbon, the cars were tuned with simpler physics, and virtually no oversteer. Relative performance of cars in any NFS game bears little resemblance to their real world counterparts.