What are some other games besides halo that has a forge?
The only other games I can think of with a Level Editor mode are the Tony Hawk games.
It depends. Halo Reach has a good campaign with firefight, custom games, forge, matchmaking, but it lacks in some game modes in multiplayer because more people are playing halo 4. Halo 4 has War games, spartan assault, a good campaign as well, with forge, matchmaking custom games. For me I prefer halo 4 but its really your decision. Both are great.
Do it like any other thing you load up in theater. After ending your game of forge, go straight to theater. It will already be loading. However, if you played something after forge, go to select film, recent games, and you should find it in there. Hit start in theater if you don't know the controls.