We have some incorrect beliefs about God, but we are supposed to be teachable and humble -> not required to have penetrating insight as a prerequisite nor are we left to figure things out by ourselves.

But I’d be hesitant to use ‘heretic’ in the sense that someone has erroneous ideas about God. There’s a world of difference between someone who has an incorrect belief about God, and someone who insists that his incorrect belief is right contra Scripture (or contra what is inferred from Scripture by “good and necessary consequence”, e.g. often the traditional interpretation, particularly in the Reformed or Calvinist strand).

Someone who likens God to three states of matter, e.g. ice-water-vapor (even though that analogy is more modalist than triune), is quite different from someone who, when being corrected as to how that formulation is wrong, keeps arguing the point and subverting the authority above him.

It is the latter, and not the former, who is a heretic. Or maybe what I’ve said just now is still going too far: perhaps he’s only a heretic if he holds the position despite church discipline.

I am only saying this because I have come to understand heresy as that which is associated with hellfire. The person who is teachable or accepts church discipline was not a heretic until the point he learned the right formula. We never know God well enough or full enough to exhaust who God is with a formula or a description or analogy or theological model.

Thus, a heretic is someone who lives in unrepentant doctrinal sin, whereas someone who makes an error unknowingly is just erroneous or ignorant of sound teaching. The former should’ve known better and continues anyway; the latter is willing to know better and hence accepts rebuke.

This guards us from saying that someone who becomes a new Christian is not really a Christian until he gets his doctrine straight, or from this implied notion that you can be a heretic and never realize it.

I don’t think the passage would mean the “believing Gentile community”. I really think that’s poor exegesis.

Similar to what you were saying, John Piper tries to defend it through cross referencing John 11:51-52 (link), saying that the same language is used:
John 11:51-52,
“He prophesied that Jesus should die for the nation, and not for the nation only, but to gather into one the children of God who are scattered abroad.”
1 John 2:2,
“He is the propitiation for our sins, and not for ours only but also for the sins of the whole world.”

I am not sure this is the right answer. I love Piper, and I enjoy his sermons, but this is not his strongest case or something I think he should be proud of. It feels like exegymnastics. Meanwhile, something I am not convinced of is that the verse is speaking as strictly as is claimed or, rather, that it means what Unlimited Atonement (as commonly presented) claims. Let me put it another way: I think all Scripture should be given weight, but this verse seems like you’d want several other passages to have similar claims. I think Limited Atonement can suffer this same criticism, but significantly less so.

Here’s my take of the verse, in layman’s terms
“He is the propitiation for our sins, and not for ours only but also for the sins of the whole world.”

His saving and defending and cleansing work isn’t for us (those who currently believe) only, since anyone can claim Him and be saved. It isn’t our immediate community. It isn’t just the first generation, or those who saw Christ. It isn’t just the Jews. It is for everyone. Given context, the propitiation is pointing to Christ’s cleansing, defending those who confess their sins, etc. This is not true of the whole world (since some are damned), but anyone in the world can call on Christ and be saved. It all hinges on Christ being our Advocate, and He is only an advocate for those who confess their sins and are cleansed, but at the same time He can be the advocate for all. He is our advocate because He is our propitiation. So, He is for us, but not us only but for everyone.

John 1:7-2:2
7 But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.
8 If we claim to be without sin, we deceive ourselves and the truth is not in us.
9 If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.
10 If we claim we have not sinned, we make him out to be a liar and his word is not in us.

1 My dear children, I write this to you so that you will not sin. But if anybody does sin, we have an advocate with the Father—Jesus Christ, the Righteous One.
2 He is the atoning sacrifice for our sins, and not only for ours but also for the sins of the whole world. 

[edit]
I think a lot of the debate comes down to already, but not yet tension; and understanding of God’s sovereignty whether God plans some things, no things, all things, or whether His plans can fail (they can’t, but some think they can) — Did God’s plan intend that all be saved? These topics’ answers are often assumed, and it is frustrating to discuss since they are so loaded.