For the silver trophy, "Level of Merit", where you need to complete all career challenges, you can actually just restart the race and each completed challenge will be saved, so going one at a time after restart works, however, it's the TOKENS that will not be saved if you restart after collecting them.
Please NO Blank Friend Requests, otherwise I will simply ignore them.
Tokens always saved for me when I hit restart. I haven't verified whether restarting resets career challenges because I don't want to lose one and not get credit!
By the way, I just read on the modnation racers forums that they are no longer accepting new submissions for hot lap mode. They are just recycling the ones they already have, so you can't get XP for Star Creator that way.
Currently working on: Cloudberry Kingdom, FFX HD
Platinums (27): 3D Dot Game Heroes, Borderlands, de Blob 2, FIFA 14, FF XIII, FF XIII-2, inFAMOUS, Jak 1, Jak 2, Jak 3, LittleBigPlanet, LittleBigPlanet 2, Medieval Moves, ModNation Racers, PlayStation All-Stars Battle Royale, Racquet Sports, Ratchet & Clank Future: A Crack in Time, Rayman Legends, Rayman Origins, Sly 1, Sly 2, Sly 3, Sports Champions, Sports Champions 2, Tiger Woods PGA Tour Golf 12: The Masters, Trine 2, WipEout HD