I have an idea for improvement:
When you have an option that has in stock level together with an option that has not, it will show something like:
Option 2 [4 in stock]
This can be confusing because it might hint that the Option 1 is out of stock while it is not. I would suggest that every option that does not have in stock level will simply show:
Option 1 [in stock]
This came from one of my customers who thought the Option 1 was not available so it's a real issue.