[Steve also trusts Natasha's instincts, and after his initial outburst of surprise, he reigns himself in and stays calm throughout her explanation.

There's definitely a lot of mystery around the situation, because why wouldn't Loki hide himself more convincingly? If it's him, he's probably trying to confuse them or at least make them second-guess what's happening.

If it's not him, then they need to make absolutely certain that's the case before acting.]

The other possibility is that it's a Mirror, but that would mean it's probably Loki's Mirror, and it doesn't seem like it's that easy for them to cross over in the first place.

Either way, we need to assume he's dangerous and remain cautious until we figure out what's going on.