Rule #1 of masking: When you practice whatever behavior you want to model, you do not do it with the object you are mimicking. For that very reason that you listed, it's hard to explain that you aren't mocking them when you're awkwardly copying them.
Rule #2: If you're especially confused about interactions, the only research you can really do is reading in order to know the expressions associated with a reaction (emotion). Then look up those expressions, bearing in mind that fat and bone structure may make it look different from face to face.
Rule # 3: Replicate effective and positive behavior. If it worked for someone else, it usually will work for you. The issue is identifying the right habit.
Rule #4: I understand that you were trying to be relateable by mimicry but you do not do that with facial expressions. You can do small gestures so that they subconciously react to you. If they turn to you, you turn to them. If they shift their arm, wait at least ten heartbeats before you mimic them.
People are annoyingly complex when it comes to understanding them. Interpretation is the hardest when we only have do much to go on. I don't know if that was terribly helpful but I've been masking for a while. After a while, it becomes second nature to respond and act a certain way.
When she said that you look like someone she could talk to, that was more that she had something she really wanted to voice (something that was bothering her and she felt she had to tell someone). That means you seem like a listener (because you're most likely quiet). She just wants you to sit there while she rants with occasional input so that she knows you're listening. Questions that relate to her story like: Why did that happen? Did she tell whoever pissed her off, can she?
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I wondered, ". . . So therefore I exist."