And yes, they could unilaterally implement something, just like they have the Freezer, and everyone else will have no choice but to follow.
The freezer, from a mathematical perspective, actually exacerbates the problem. What could BMG possibly implement (other than unplugging the freezer) to alleviate the problem? They’re not in a position to either force boys to marry earlier or girls to marry later.
What statement of mine indicates that I disagree with your assertion that poorer girls, from kollel families, are less likely to get married? While we’re at it, do you have any evidence of this? (Yes, I know those two points seem contradictory, but I haven’t yet really consider it from that perspective.)