1: So let the 19 year old straight out of Sem children be supported and live by Mommy and Daddy for another few years before they move out. It seems like the only reason that support is needed to be discussed is because the couple isn’t really ready to get married, which then certainly the parents are going to be in charge of their “married” children (who are Somech al Shulchan Avihem, with the halachic ramifications thereof?). In that case, of course the parents should discuss support, no different than sending your 10 year old to camp and expecting them to pay for it!!
2: Shouldn’t how much money is “enough” be a decision of the couple, not one that involves the parents?