After long time with xenforo and it's experience the code of threadthumbanil is not shorter 150lines to 40lines and lighter without too much template modification. removed multiple fucation to check the image url and attachment
FIXED : Now it will select attachment which image only not the first attachment only
ADDED : Default thumbnail Url if there is not image and attachment thumb
you mean Latest posts ? well not yet but gonna add that too in the next update
you might didn't read the wordWidget: "New threads"
Display style: "Simple"
I was able to find the regex (find/replace) of your template_modifications.xml file in the file "thread_list_macros" so I'm unsure about what could be wrong.
Fix : Thread width error fix when creating category
ADD : Thread thumbnail in New Thread widget
ADD : Thread thumbnail in the Search result