AP casts Gideon, Ally of Zendikar, immediately +1s it and declares it as an attacker. NAP objects and they call me. AP asks if summoning sickness applies to planeswalkers and I explain that it does. But what about Sarkhan? Well, Sarkhan gets haste, Gideon does not. Misunderstanding solved.

AP now asks if he can “take it back”, by which I assume (uh-oh) he means the +1. I agree, since he only did that because it was his intention to attack. He then picks up Gideon, puts it back into his hand, and proceeds to cast another spell entirely. His opponent nods in agreement, so I decide not to press the issue.

Thoughts?

At Regular I'd probably stop him putting it back in his hand. I would have thought like you did - he wants to use a different ability. And I don't like using “the opponent was ok with it” as carte Blanche for letting players do what they want. Maybe on this instance it's fine, but not for me as a general rule of thumb. While we're encouraged to fix games with plenty of leeway, I don't like the idea of allowing complete take backs just because you didn't read the card.

Undoing the ability, I'd check with the opp first but be fine with it overall. It's a learning experience either way but I'm not a fan of giving someone a huge break on my word alone because they didn't read a card. While it may be a feelbad for AP if they don't get to take it back, I can imagine it could be a feelbad for the NAP if they get a free takeback due to ignorance.

Taking back the spellll…. I would not support that, personally but If the opponent is allowing it, I have little reason to insert myself in to the situation. I'd likely make sure the opp is fine with un-casting gideon but not impede them further.