You can't. You just have to either accept it or stop using commands and be a real Minecrafter.
Yes there is 2 I know of first one is the latest edition of TooManyItems and second is singleplayercommands
You can only turn off the rain if cheats are enabled. If they are, the command is /toggledownfall
The button "O".
/gamemode a
/gamerule mobGriefing false
You press E or Escape on the keyboard.
turn off the firewall is all you can do.
You can't turn off the timer since it's a demo game.
To turn off chat in Minecraft multiplayer you can type /ch off in the chat box. This will disable the chat feature for you and you will not see any messages including join messages as well as private messages.
No but is you get Optifine you can
You change your game mode to survival or adventure
change your render distance on to tiny turn particles off and smooth lighting off