While multiple instances of the widget are being created with user-defined settings, these are the ways that do NOT seem to work when trying to insert a .css file based on the instance style setting.
1. Register, enqueue then print a link to a style file in the header for THAT widget instance while it is being created.
2. Create an array, make it global in the class, save the css file name to the array like:
$style_arr[$style_name] = $style_name;
then, after looping through all instances of the widget, loop through the style array and insert the required styles into the head. While the global array will save the style names, there does not seem to be a way to call the wp_print_styles function after the style array has been populated.
Currently, I am just printing out all possible styles in the head, regardless if they are needed or not. Clumsy, not efficient and not at all flexible. Anyone? Bueller? Bueller?