Is staying out of it a position?

Yes, and not the one people usually assume.

Silence is legible. When something is being argued about in front of a group, everyone in that group is doing something, including the people doing nothing, and observers read all of it. That part isn't really in dispute.

What silence doesn't do is carry content. It registers as a stance without specifying which stance, which means whoever is reading it supplies the rest out of whatever they already believed about you. Someone who expected you to agree reads confirmation. Someone who expected you to object reads disappointment. The same silence produces both at once, in the same room, and you don't get a say in the split.

So the honest answer comes in two halves: it functions as a position, and it is not a position you chose, because you didn't choose one.

Because of something that happens at the level of the group rather than between you and any one person.

Elisabeth Noelle-Neumann named this the spiral of silence in 1974. Her account: people who sense their view is the minority one become less willing to say it out loud, which makes that view look scarcer than it actually is, which makes the next person holding it quieter still. The estimate and the silence feed each other.

The part that matters here isn't about you specifically. It's that people gauge where a room stands by what the room says out loud. An unexpressed view is, for the purposes of that estimate, simply absent. So silence doesn't get read as agreement so much as it fails to register as disagreement, and inside an argument those two land in almost the same place.

That's a mechanical fact about what is visible. It isn't a claim about what anyone owes.

Yes. Unambiguously, and not grudgingly.

How a person uses their own voice is theirs to decide, and the reasons for staying out of something are frequently good ones. It isn't yours. You don't know enough yet. You'd be talking over the people it actually happens to. The cost of your speaking lands on somebody who didn't get a vote. None of that requires a defense.

The right and the reading are just two different things, and neither one settles the other. Having the right to stay quiet doesn't come with a guarantee about how the quiet gets interpreted, and noticing that is not the same as being told to speak. It's information about what silence does and doesn't do, which is what you'd want before choosing it on purpose rather than finding out afterward that it said something.

Related: Am I keeping the peace, or just avoiding the conversation? — the same question inside a conversation you're already part of.