If I did feel (I probably wouldn’t, but I can “step in your shoes”) that I wanted to make a point with her (rather than a scene with her, which is what almost happened, it seems), I would take that after-Mass opportunity or some other church social opportunity to take her aside – such as at a church pot-luck? (we have these sometimes). I would bring her a dessert plate as an excuse to start a pleasant conversation with her, introduce yourself, tell her how delighted you are that she is one of the many who come early to pray the rosary with others. After “killing her with kindness,” I would gently mention that you understand she “really likes that seat in that pew.” That will probably get her to respond. She’ll either reinforce that (maybe vehemently), or ignore it. She might even deny it, out of embarrassment. In any case, a gentle remark might help her to see that her behavior has a strong effect on others. People don’t always respond interiorly in the same way that their public face reveals. The incident was public. She had a public “face” to protect. Perhaps the best way to be a non-enabler is to do so after-the-fact in some way like this. Making her aware, plus praying for her, will probably be more effectual in the long run than “standing your ground.”
People with bad manners or a heavy personal agenda usually have some misery in their lives that needs healing. Your example of gentleness may be more of a graphic contrast (a “teaching moment”) than a lecture in the presence of the Blessed Sacrament.
This is not said with the slightest bit of superiority. I fully admit to having a tendency to territoriality

, but at least being good-humored about it.

We make jokes after Mass about each person having his or her “assigned seat.” I’ve seen that in adult classrooms as well. (I agree with the amused priest that another poster referred to.) I have a “thing” about not being crowded. More than “assignment,” I need space. (Hey, Jesus went “apart from the crowd” to pray.

) So when it’s not truly necessary to squeeze into a row (but we’re practically getting intimate with each other

), I move quietly, without incident, to another pew, with a little more room.
Bottom line: I agree with the apparent consensus here: let go and let God, as they say. If you feel you need to make a point with her about the offense she caused you, I would do so not in church, and not in any publicly embarrassing setting, and preferably non-confrontationally, because confrontation didn’t work the first time. Perhaps joking about it (without viciousness) would do more. Humor can be a powerful vehicle, when used well.