Well, that's a fairly hard thing to do, for several reasons.
First, you have 4 slots, lance takes 2, 2h sword takes 2, shield takes 1, 1h takes 1. So you'd always have to leave one item away, with one exception. There's a couple of swords that can be used both 1 handed and 2 handed - Longsword, bastard swords, katana, langes messer (this last one only takes 1 slot, as it starts as 1h weapon, you must switch into 2h with x).
Secondly, items must be repaired and heavy horses are extremly expensive. So the best idea for a heavy horse rider is imo, only use 1h and shield to have lower costs.
So imo you should first try one of the styles, you can make a second character for the other one. It is a better idea to make such a character when you have lots of money, all the needed items and know the game well. There's a couple of guides for 2h and riders in the beginners questions section.
a) Lvl 30 is the start of the soft cap. I wouldn't plan on higher.
c) Swing speed (and damage) is increased by weapon skill (WPF) (for each weapon type, you get more points by buying weapon master). Agi (and athletics) increases movement speed, which is often used to swing faster by moving with your swing. Powerstrike (PS) increases your damage with all melee weapon types.
b) There is no magic number. My personal lower limit to make it usuable with 1h swords is 6PS and 50wpf, or 5PS and 100wpf, I'd go for at least 6PS 120 wpf though, 2h isn't much different. At 130 wpf you can raise it only so little, you can easily split. Lancing needs not much, as speed adds to damage.
d) afaik, no.
