I like to employ a little courtesy-based psychology. If I confront someone in my assigned seat, I look at my boarding pass and then look down at the person in my seat saying, "Darn -- They must have double-booked 6A." I think this gives the person some face-saving wiggle-room. More often than not, they will look at their boarding pass and say, "I'm in 7A." I'll say, "You're close, but you missed by a row. No need to get up. I'll take 7A." (Admit it, folks, we've all sat in the wrong row at least once!) By "blaming" the airline's computer, this also gives me the opportunity to invoke the FA's help without escalating the situation.

In the case cited above, I would definitely call for the FA, since there would be a complex move involved. (For the moment, I'm not considering the rude & self-centered approach the mother took.) I've heard FAs announce that they were trying to sit a family together and would some folks like to move? As someone else mentioned, there are often people who are desire location more than seat position (front middle more desirable than back row aisle).

Also, if I'm seated in a two-seat row and a couple winds up with one spouse next to me and the other a row or two away, I'll say to the person next to me, "Would you guys like to sit together?" More often than not, they booked at the last minute and they would like to sit together, but do not want to impose upon people by asking to switch.

The only time I've ever been overwhelmed have been situations in which I wound up in the middle of high school kids on a trip together

It's all about everyone trying harder to be civil towards one's fellow humans who share the same planet -- even entitlement-bent soccer moms as described above.