I doubt you'll ever be able to get a debugger setup to run with calibre. But then debuggers are highly overrated when it comes to python. I have never once used a debugger in the last 4 years of calibre development
Use the following trick to get to an interactive python prompt
from calibre import ipython
ipython({'myobj'
bj})
where the dictionary contains the names and objects that you want to be available in your interactive session.