The inside surface would be a mirror image of the core. The smoother the core, the smoother the insides. However, I would not worry too much about the air flow due to the surface. The main air flow issues will come from the bends. Make the bends as gradual as possible and that will ensure the least amount of of pressure drop.
As far as ensuring that all the EPS is gone, that's easy. A small bottle of acetone (finger nail polish remover) will dissolve at least one cubic meter of EPS. Plus it works fast. Just keep recycling it back through the snorkel until you're sure.
As far as ensuring that all the EPS is gone, that's easy. A small bottle of acetone (finger nail polish remover) will dissolve at least one cubic meter of EPS. Plus it works fast. Just keep recycling it back through the snorkel until you're sure.