so how do you determine whether the loop is an infinite one? and who will pay the numerous custodians in the galleries needed to click on "yes, i really want this loop?".

You have a point. Better stated, you are absolutely right. The second solution I mentioned is definitely better. Which was more or less the same that you mention:

that it would be nice to be able to halt the virtual machine without quitting Pd.

Anyway Pd should at least always allow you to save patches and gracefully quit.

it would also be nice to have a step-debug mode...

I would be happy with just being able to find not-only-the-last error