Have you thought about appropriating a few? Appropriating, but asking the homeowner first, of course.
You mentioned your mother's palmetto that seems to have not grown much since 2011. I'll bet I have her beat. My palmetto was planted in 2007 as a strapleaf bareroot specimen. Today, it is still alive, but still no trunk! Not even CLOSE to trunking. So, what have, in practical terms, is a Sabal minor with costa-palmate fronds! But you know what? At my latitude and for the severity of winters I get, I'll take a permanently stunted (but alive) palmetto.
I have heard through third party observations online that palmettos exposed to near-lethal cold will often do this. Something clicks in them where they forever give up their will to grow a trunk, but will stay alive long-term as non-trunking palmettos. I wonder if it's a random thing or if it's an intentional strategy. Perhaps a palmetto brought to near-lethal temperatures PURPOSELY gives up a trunk so that the terminal bud can stay close to the soil line and derive the benefit of a degree or two more warmth since it realizes it's in a cold winter location.
Might this have happened to your mother's palmetto?