I will prove to you that God exists

[Angrboda]

I'm not invoking any specific explanation.  I'm pointing out that we simply don't know one way or another whether the characteristics could be other than what they are.  Since any argument that a creator had to tune such things requires that they needed to be tuned, no such argument can get off the ground without some evidence that they would have needed tuning.  It is a key premise of the argument from fine tuning that they could be or had to be tuned.  If there's no evidence for this, then it isn't a sound argument for God.  Since it has a material impact on the argument and issues in question, it is not a red herring.  Though I will welcome any explanation from you as to why you think it is.

The key point is that they are tuned for life to occur. Even if the universe "had to be the way it is" for some unknown reason (such as being intentionally caused to exist) its still fine-tuned for life. You're offering an objection you admit is only a mere possibility which I seriously doubt you subscribe to.

The word tune means that they are adjusted to a specific value.  If the characteristics could not be other than what they are, then not only were they not adjusted, they indeed could not be adjusted.  You are simply using the word 'tuned' incorrectly.   If the universe's characteristics cannot be tuned, then they very well can't be fine tuned.

You're missing the point of the objection which is that in order to argue that the values are a product of design then it is necessary that they could have been other than what they are.  If you have no evidence to indicate that they could have been tuned or needed to be tuned, then there's no reason to suspect design and the constants being what they are is not evidence for God.  You either need to provide some evidence that the characteristics could have been adjusted or accept that they are not evidence for the existence of God.  Just like a murder trial, if I raise reasonable doubt that my client committed the murder, by showing that you lack a critical piece of evidence, I have invalidated your case against my client.  I don't need to prove that someone else did it, only that you didn't prove my client did it.  Thus I need not show an alternative to God fine-tuning the universe if you cannot demonstrate through evidence that it even required a God or other force to adjust the values.  This idea that I need to subscribe to an opposite theory to demonstrate that your theory doesn't work is simply an irrational idea of yours.  It truly is a red herring.

Red Herring
a fact, idea, or subject that takes people's attention away from the central point being considered:

Quoting the definition of a red herring does not explain or demonstrate that I am guilty of a red herring.  Are you really this stupid?