My favorite putty is Bondo spot putty which can be found at most auto parts stores.   It cures up within an hour, and is softer than the plastic, which means you can sand it pretty easily.  It doesn't shrink, and does bond to the plastic pretty well.

I don't know what you used for primer but there are some pretty good paint strippers out there.   Castrol super clean is the best one I have used.   I have also used 90% alcohol as well.  Neither of these will damage the plastic, nor remove glue, but they might take off the putty.

I too have wondered what was inside the box for this kit.  Thanks for showing us.