// I had previously used a CSS/JQuery extractor to get a URL from a page and add it to JMeter vars - accessing it here
var pageURL = \"${valueFromJmeterVars}\";
// navi
I believe that you need to cast it to String first, as per Using Selenium with JMeter's WebDriver Sampler guide JMeter Variables are basically Strings and you can't put boolean there.
just replace
vars.put("reserveASpotButtonIsEnabled", reserveASpotButton.isEnabled());
with
vars.put("reserveASpotButtonIsEnabled", reserveASpotButton.isEnabled().toString());
And it should work.