A late reply, but always helpful for anyone who finds this website:
1. There is no cheat to get the Grim Reaper. The only way to become a Grim Reaper is to be the player with the most points (of the entire game, not just your own color), and be on the winning team. If you've got the most points, but are on the team that lost you'll be a giant chicken (just as strong and powerful as the Grim Reaper). A nice tip to get many points very fast: if a player dies, and leaves a ball of the opposing teams color behind, pick it up. You'll switch to the other team, but also get 300 points for doing so.
2. To use the new classes you must pick up the sparkly wizard hat. Not the Mage hat (the regular one), but the purple hat with the yellow stars. It usually appears close to the throne. Sometimes there's a timer counting down untill it appears. When you pick up the hat you'll be a ninja, pirate or giant at random. By using the triangle button you can switch to another role (of the 3) for a limited time after you picked up the hat. This is indicated by the traingle sign above your head.
3. Teamwork. But sometimes your teammembers don't seem to do much of it. For you to gain points (for Grim Reaper or giant chicken), and help the team here are a few hints: in a new game start with a worker, and first try to get as many ore as possible. Try to upgrade the worker as soon as possible, so you can defend yourself while obtaining the ore from the main ore supply (which the enemy also wants to get). Then upgrade the other classes, and get some extra wood to make new castledoors in case the enemy destroys them. That way your team will be strong very fast. If you manage to get the main ore supply before the enemy does, you'll have a nice advantage. Next get either a healer to heal you teammates who are attacking, or get a class with which you can do plenty of damage to attack so you slow down the enemies attack/defense, and take down the enemies doors. Or get a strong or sneaky (ninja) role so you can get as far into the enemy castle as possible, to try and grab the princess.
4. I'm not sure, but I don't think you do.