can we use a two handed sword with quick swipes on one character and then use the same sword in large, fatal swings on another?
Use exclusively power attacks on one character, none on the next. Or just use dual-wield weapons if you want speed, that's what it's here for.
are there any variations in magic builds? can two people make lightning centered destruction characters and end up with different feeling combat?
It looks like Chain Lightning might be chargeable, so yes. Not to mention the fact that there's five distinct schools of magic you can focus in.
will my archer have the option to shoot multiple arrows / reloading faster / traps and technology / ammunition varieties?
I expect there to be some kind of perk which increases reload speed. Alchemy's poisons add variety to ammunition. There's some trap-like spells you can use.