CB and PR, you both have this concept down very well. I can't really add much, but CB, you are very astute when you say that "marking" is the beginning of boundary-setting. And that the emotional management that a healthy mom does with her children is the precursor to both personality and the way that dyad manages their boundaries throughout the lifespan.
An example of mirroring and marking would be when a baby cries, you make sympathetic sounds (oh, it's OK baby, yessss, I know you are hungry ... in an appropriately sympathetic tone) but you can look into their upset face or cuddle their tense body and not feel yourself getting tense and anxious and upset in response. You remain calm, and radiate calm with your manner, even though you may stick out your lower lip like the baby and furrow your brows like he/she does. It's similar to when somebody "puts on" an upset facial expression but you can tell they are not really upset, that it is "put on." So, the baby sees that you see how he/she feels, and is mirrored (he exists), but his emotions don't cause you to get upset too. His emotions are his and yours are yours. The first rudimentary form of a boundary.
In my mom's case, she would sing to the baby and jiggle her a little, and her voice would begin to quiver a little if the baby didn't calm down immediately. She was getting anxious and it came out in her manner even though she was trying to do the right thing. I think this comes from her own infancy --- that time when we learn that just because someone else is upset, doesn't mean we have to be.
Allan Schore wrote a thick book on the neurobiology of the mother/infant attachment relationship ... I wish I could remember the name right now. It was a really hard read ... but basically he condensed down a lot of neurobiological research and said that the mother acts like an "external frontal lobe" for the infant, as his/her own frontal lobe is relatively undeveloped. The frontal lobe is responsible for "executive function" --- the control room of the brain. The mother has to regulate the baby's emotions because his brain is not yet developed enough. So when he is getting too upset (say, upset enough to spit up his milk), the mother --- or grandma, or dad --- must hold and soothe and reassure him that he is OK. Or when he is sluggish and inactive, the mother --- here it's more likely the dad or the grandma

--- comes into his field of vision and engages him in play that ramps up positive emotions.
More later, Pilgrim