1. The start date and time is in place so everyone is on a even playing field and has the same amount of time to make their game. The core of your game should be made within this timeframe. You can establish a base framework (see the templates on the resource page) and I can't stop you from thinking about what you would like to make since a theme is not enforced. But in the interest of fairness everyone should make their game within the time period specified.
2. That particular line is talking about game engines. A game engine that has a set resolution can still be used if you can find a way so the player only sees 84x48 pixels of the space. If you're using a "mainstream" engine like GameMaker or Unity you don't really have to worry about this as you can just set the games resolution/camera viewport to 84x48.