All characters are different and better in their own way. It is best to read through their attributes and select a character that you think will suit your playstyle. Play around with it a bit and decide from there.
Getting your character lodged into places is an unfortunate occurrence in Bethesda games and the only way to become unstuck is to reload. Luckily, Skyrim offers about three autosaves, all very current, that you can use to load from.
as long as you have 2gb of ram, which is how much Bethesda have restricted Skyrim to use on any computer ( without the use of a mod), you should be able to run Skyrim, mediocrely
skyrim
No shadow is not a character simply because They decided to use him as a Assist Trophy.
Use a Khajiit, they are good at sneaking.
You can marry any NPC character in Skyrim through the use of console commands. With the exception of Serana from the Dawnguard DLC (There might be a way to marry her, but I haven't found it). Vivienne is not a character who can be married without console commands.
You 'use' RAM, not lose it.
Get the conjure familiar spell and use it
There are no such actual set "rules or guidelines" it is merely how you play the game and what path you take your character down. Just use common sense when playing.
Equip arrows and then use poison.
Yes, all playable races can use shields.
If you have Hearthfire installed you can use them to make Flour from Wheat.