Ron, if you keep the base, I think can kill the buggies and larvae by kiln-drying it; this base is small enough to use your oven as a kiln.
(The chemicals are NASTY and not always effective; trust me, I know from a bad experience... :-( )
Perhaps Uncle Skip/Glenn can chime in here as to an appropriate temperature and duration?
One the buggies are dead and gone, you could fill and restore the rest.
Good luck! I know you will find a good solution. And I really don't think dude was out to hose you. I hope you end up feeling good about it :-)