Just curious, what part of this is a scam considering SLEC doesn't charge additional for this so there's no profit motive because they earn less when this happens.
What they might be guilty of is being too cautious.
What is the current status of petition on CEAC? Hopefully it still says READY.
You probably should be in periodic contact with the Consulate. They have the ability (but not the obligation) to extend the petition approval for up to one year but if you don't inform them they will close the petition and send it back to the states. If this happens you will need to start over.
Hundreds of people on here have done follow to join. You begin with the Consulate stage of the process that the k1 previously did. So yes, the DS160 is the starting point.
The interview age varies by consulate but age 14 or older is the most common.
Since you are preparing the DS 160, Don't pay attention to the IR/CR visa at it would have been a decision you would have needed to make a year or more ago
The petitioners marital status is not an issue for the approval of a parental visa therefore no requirement or reason to show the marriage certificate.