Maybe this is how Cadence got Shining to go out with. Or maybe Twilight, the pony who can learn any spell just by watching somepony else (having seen her do a dark magic spell after watching Celestia do it in Crystal Empire episode, this is confirmed), watched Cadence perform the love spell, as she did in the flashback about her, and cast the spell on Cadence and Shining Armor, so that her favorite foalsitter and BBBFF would be together. My own theory/headcanon. Someone should make a comic of this.
Not necessarily. Perhaps it is just like a skill/talent, for some it comes naturally, others have to work at it. If someone really studied it and tried, they could do it. And if they had the power, which Twilight does, being the element of magic. So I think she could learn it.