1) Yes. You get a discount on Sponsored Magic if you already have Evocation and/or Thaumaturgy (per the rules), but they are not required. Some other Sponsored Magic options have different rules (like Kemmlerian Necromancy) but Seelie/Unseelie is your garden-variety example of Sponsored Magic.
2) Prolong Spell is basically an unnamed fifth spell option (in addition to Attack, Block, Maneuver and Counterspell). It is a spell you cast *after* the Target Spell you wish to prolong (for example, an Armor spell). You determine how many shifts of duration you want to add (X), and that becomes an X-shift spell. You pay Stress as if it was a normal X-shift spell, roll Discipline to control it as a normal X-shift spell, and when you succeed, your Target Spell (Armor spell) is increased in duration by X shifts.
3) I believe so, yes.