The double-slit experiment works with quantum objects, and no macroscopic object can be a quantum object. Not even an atom is a quantum object.
What these other objects, like Bose-Einstein Condensates, superfluids, superconductors, atoms and even lasers, are, are quantum systems.
While they may exhibit some quantum behaviours, they cannot behave exactly as a quantum object would and are not compatible with experiments designed to show those properties of quantum objects.
The wavefunction is not a concrete phenomenon, like a wave. It is an abstract, a mathematical representation of all possible states of a quantum object (or property). The quantum object does not "exist as a wavefunction" when unobserved.
What in Gods name are you on about boy?
Atoms behave quantum mechanically as do photons and all other subatomic particles. Objects normally behave quantum mechanically up to a few hundred atoms in size. A buckyballs contains 60 carbon atoms and the dual-slit experiment can be done with photons, atoms or using it the buckyball if you wanted (as you will know if you actually bothered to read the link I gave you).
Larger objects can be made to behave quantum mechanically by preventing heat loss. Under entropy heat loss is the leakage of information which collapses a quantum state. By preventing heat loss scientists have been able to get an object which is so large it can be seen with the naked eye into a superposition of all states. It was a peice of wire.
As for wavefunctions you are wrong there too but I have written enough. The rest is up to you.