I agree, but I don't think #3 is possible. For a variety of reasons, both biological and societal, the older people get the less likely they are to have kids, and fewer of them.
Kiryas Joel, New York, has astronomically high fertility, but also extremely low rates of under-18 pregnancy - which proves the combination is possible.
That is an exceptional outlier. It's almost entirely (>90%) made up of one religious group that has strict cultural rules about early marriage and having many children.
That's not something typical outside of that one particular group, nor can you copy-paste that anywhere else.
I think in fact it's the opposite. Because of how universal below-replacement rates are in EVERY developed nation, it shows just how independent it is from factors of culture, economics, environment, etc. It takes a seriously unusual situation like the one in Kiryas Joel for below-replacement to not be the norm.
I disagree - very high fertility combined with very low rates of extramarital pregnancy are totally normal for traditionalist religious groups-not just ultra-Orthodox Jews, but also the Amish, Salafists, Deobandis, vilayet-e faqih-e faqih Twelvers, the SSPX, inter alia - what Eric Kaufmann (see his book Shall the Religious Inherit the Earth?) calls “endogenous growth sects”. There is nothing unique to Kiryas Joel about it; Kiryas Joel is simply the most easy-to-point-to example of a wider phenomenon