Yes, you can perform all five assuming you get a high enough skill level for each school.
Of course there is.
Wrappings, spells, and ritual.
You cannot construct your own spells as you could in oblivion, you must find or purchase spell tomes and consume them.
A witch-doctor performs a ritual by usually casting spells and curses. They first start off with a sacrifice then continues by reciting some spells for the specific situation.
There's a student who walks around in the collage of Winterhold. He specializes in illusion spells, and can train you, and sell you illusion spells.
they are scatterd all over skyrim but you could buy them from Phines Gestor he is in the collage of witerhold located north of winterhold
Destruction Spells.
It depends on your own choice, if you want to fight dragons, complete missions and cast spells go for Skyrim. If you want to assassinate guards, explore Rome and Italy or shoot cannons go for Assassins Creed. Personally I enjoy both but Skyrim does get a bit boring further into the game, but I could also say the same for Assassins Creed. It really depends on what you like.
A coven is usually a group of 13 witches! They cast Spells in a ritual and ceremonial way! The best site for spells is http://www.moonlightspells.com
'Found' spells are typically randomised so it's very, very hard to direct anyone to a specific place. Also, they tend to be novice-level. The best place to buy them from are from the Jarl's mages, and the college at Winterhold. Firestorm is a Master-level spell and is only given to you after performing a certain ritual at the college, but this option will not be offered to you until you have 100 in Destruction. Same for the other schools of magic and their Master spells.
There is no option to make custom spells like Morrowind or Oblivion. You have to stick with the ones taught to you by the spellbooks you buy.
Yes, there are no classes. Any character can learn spells and start developing magic skills.