Quick answers
How do I use Dead Eye on PC?
Aim with Mouse 2 and press Caps Lock. Paint targets by moving the cursor over them (or hold Mouse 1 on later levels) and fire to shoot them all in sequence.
How do I make my horse gallop?
Tap Left Shift (Cross/A) repeatedly to build speed; holding keeps the current pace. Gallop drains stamina and can throw you if the horse is spooked.
How do I open the item wheel?
Hold Tab for the weapon wheel and press Mouse 3 to switch to items and horse gear; on controller hold L1/LB and press R1/RB.
How do I set a waypoint and let the horse ride there?
Place a waypoint on the map (M) and hold V for cinematic camera while riding; the horse follows the road until you touch the controls.
Why does E do so many things?
It is the context key: mount, greet, loot, open doors and pick plants depending on what you look at. Aim at an NPC with Mouse 2 to get the greet, antagonize and rob options.