Can I play Back Off on my own Irelia Fervent and defy it myself (or abandon or hard bargain, doesn't matter), resulting in the spell not stunning Irelia BUT receiving a +1 from choosing her?

Yes — Irelia, Fervent keeps the +1, and Back Off gets countered.

When you finalize Back Off and choose Irelia, her "when you choose me" trigger fires immediately and goes on the chain above the spell (the trigger is independent of the spell, so countering it doesn't undo the +1). You can then play Defy (or Abandon/Hard Bargain) targeting your own Back Off — nothing in Defy restricts who controls the spell, and Back Off's 3-energy/0-power cost is within Defy's limit. The chain resolves LIFO: Defy counters Back Off (no stun, and no draw from Back Off's text since it never resolves), then Irelia's trigger gives her +1. Note Deflect doesn't matter here since only opponents pay the extra rune to choose her.