1. Nothing apart from your map taking longer to compile in the VVIS step. Theoretically, more vis leaf cuts mean a more optimized map, but it's a balance with compile time.
2. No prompt, your map will just crash. There is an entity count in the compile log though (edict count). It's very unlikely you'll get there unless you're making something very entity heavy like trainsawlaser.
3. See above. However remember that the game also creates lots of entities during gameplay for the players, their hats and projectiles, etc.
4. A "window" between two completely sealed areas of your map. Because it requires each area to be perfectly sealed from each other, it can completely unrender the other area on an input. Areaportals also have the innate ability of being culling windows, where anything you can't see directly through the areaportal will be culled as well. They're a great way to gain lots of optimisation in your map even if you don't use the input.
5. Most people use the snow textures from barnblitz or viaduct which you can find under searching snow in the texture browser. As for the blends, they're fairly easy to make yourself from two existing textures. Here's a tutorial, an old one but the method is still the same
View: https://www.youtube.com/watch?v=aK9MRl4WQpw