Single player commands are needed.
In single player you cannot open the chat box without mods. In multiplayer you press 'T'.
WorldEdit
You can't make a "single player just for mods." All of your data is stored in the exactly same place and mods affect every single world. If it is a mod you can turn off then there is your answer.
Don't do that! You could get banned from the server as most servers dislike mods. Just be on the safe side and only use it in single-player
It is impossible to obtain the "MobSpawner" block without the use of mods or inventory editors.
There are single player mods that enable this as a command. But most are for multiplayer if you are searching the plugin: WorldEdit is the most chosen
you can get minecraft mods and textures off the minecraft mods website.
Survival and creative single and multiplayer
You can still add mods using a different launcher through Demo/Single player you just can't join multiplayer servers. You have to purchase a premium account for that.
By using single player commands. being OP on a server OR by placing a mob spawner, achievable through mods/mcedit.
bearshare is not required to get minecraft mods.
Yes, most Minecraft Mods are updatable.