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The UCSC article recommends hormone supplements rather than pregnancy. The TAMU article is about alpha-fetoprotein, and says that its effect is most pronounced in women who become pregnant before 20. That article says that if their theory on the protein are correct it may, in the future, be administered to women to reduce breast cancer rates. The Discovery article explicitly states that these are all just hypotheses; it also says that both pregnancies and breastfeeding (which delays resumption of menstruation) seem to produce the protective effect.
The first and second articles you cited both support what I said, which is that Seasonale or a similar birth control pill regimen will have the same effect as pregnancy. The second article talks about a protein which is not currently commercially available, but if its benefits are proven I would guess that it will become as common as iron and calcium supplements.