Hi, and congratulations
About replays, the feature is planned and Raydium, the game engine behind ManiaDrive, now supports replays since a few hours
But it's very easy to fake a replay too, so it will not be used as proof, but only to allow players to watch their best sessions and to share it. It will also allow to create cool video with the game.
To complete the last track, you need nothing more than usual. But there's a small trick with the physics and the way "boosts" works. Just keep it trying, you'll find it