Answer
No.
But it adds more ablilities to the game for the race you have it for, also they include some cool stuff about the history of the race and things like that.
Answer
Yes or at least have access to one.
Not having a codex is like showing up to a sports game with only knowing half the rules and with too few players. The Codex tells you how many points things are and what the limits are on what you can take. It tells you how you can build your army, and since your army is half the game that is kinda important.
You can get a reference sheet from the games workshop website, go to http://www.games-workshop.com/gws/content/article.jsp?catId=cat410004&categoryId=1000019§ion=&aId=3400013 and choose your team from the list. if the link dosent work go to the website and click on 'gaming' then go to articles, the downloads and finnaly 40k reference sheets. each one is free and shows every armys attack, armour save. etc, it saves alot of money
In leagues and such things you generally cannot, but between friends there are no such limitations, although some of the rules may need updating for the codex to function as well as a new variant/ version/ publication.
AnswerNo. There is soooo much stuff in the rulebook that isn't in the codex.
Front Armour
This rule can be found in the Warhammer 40K rulebook (or Tyranid codex), but due to copyrights we are not allowed to answer this question.
Read the codex
If it is in the Warhammer 40k codex for Daemons, then yes. If not you can always substitute the model for some other Slaanesh daemon.
In the codex it will tell what is allowed.
yes. they can use any weapons that are allowed in the Codex: Space Marines.
To my knowledge no, but I don't know the Ork codex very well. I do know they can use "looted wagons" though.
It should say so in your codex, but to my knowledge it is 10
Defineitley 40K.
Yes, they are one of the most prevalent sets of models used to play with the Imperial Guard Codex.
To prompt conversions made by the player/hobbyist.