Regarding hardiness of different ecotypes/provenances, I can say that I had a Bald Head Island palmetto die on me after 1, maybe two winters in ground here. I've had a Port Orange, Florida palmetto survive from 2007 to present day.
These results were not attended with the strictest scientific controls, however. The Bald Head Island palmetto was planted out in the center of my back yard. The Port Orange palmetto is planted inches away from my home's foundation, south-facing. Even though the palmetto has lived some 13 years or so, some winters it's gotten really lit up! Total defoliations with just the spear showing green. This hard life has stunted the tree's growth trajectory. After this many years, it still has no trunk, nor anything resembling one. It's basically a 3-gallon, costa-palmate Sabal minor in form and habit.
The other subject has to do with slowing/controlling growth rate of Washies.
This is just a hunch, mind you. Grow a Washingtonia in a 5-gallon, STURDY, thick container. Make sure the drainage holes are small and not numerous. Then plant this -- pot and all -- in the ground. Make sure that the lip of the pot is not buried below the soil line. Once the palm becomes rootbound in the pot, growth will slow markedly. Some roots may make their way out of the drainage holes, but these shouldn't matter too much. My guess, and it's only a guess, is that such a Washy will top out at 10 to 15 feet max. You'll still get new fronds, but the plant will be in "replacement mode", not "crazy growth mode." Might contribute to winter hardiness as well.
It's just a hunch.