Why don't the internal electric and nuclear forces holding atoms and molecules together cause decoherence during a buckyball double-slit experiment? My understanding is that anything that qualifies as an "observation" or "measurement" will cause the "fuzziness" of the superposition to disappear to the local observer, and the results of said experiment will appear in the classical form that we are intuitively used to.
So let's take the double slit experiment. I'm told that we have had success performing this experiment with larger and larger particles, including buckyball molecules. My understanding is that anything providing the "which way" information constitutes a measurement. Am I correct in my understanding that blocking one of the slits, trying to use magnetic detectors, or trying to fire lasers at it, perturbs the behavior of the molecule (in this case), such that the outcome of the experiment is affected (the interference pattern, as it were)?
So if it is that the interactions with photons from lasers, or the fields of magnetic detectors, make the buckyball molecule behave differently. Why is it then that the forces holding the molecule together (I guess the electron sharing or covalent bonds?), and the forces holding the atoms together (I'm guessing electromagnetic forces between the electrons and nuclei, and nuclear forces within the nuclei?) are insufficient to cause decoherence? Wouldn't this suggest that it is not merely these interactions/forces that cause the perturbance which causes decoherence? Are there any forces or interactions within the molecule that could cause decoherence?
 A: 
My understanding is that anything that qualifies as an "observation" or "measurement" will cause the "fuzziness" of the superposition to disappear to the local observer, and the results of said experiment will appear in the classical form that we are intuitively used to.

The mathematics which controls quantum mechanics describes observed "fuzziness" by using complex variables which introduce in the description of a physical state phases . The predictions of the theory, which incidentally fits data very well, are probabilistic, i.e.after many measurements will show the probability of finding a "particle" at (x,y,z,t) with an energy and momentum four vector. That is what fuzziness is about, the sinusoidal format of the wavefunction whose complex conjugate square gives the probability distributions.

So if it is that the interactions with photons from lasers, or the fields of magnetic detectors, make the buckyball molecule behave differently. Why is it then that the forces holding the molecule together (I guess the electron sharing or covalent bonds?), and the forces holding the atoms together (I'm guessing electromagnetic forces between the electrons and nuclei, and nuclear forces within the nuclei?) are insufficient to cause decoherence?

The bucky ball experiment is in the realm of quantum mechanics because of the small dimensions and momenta involved. The interactions with photons in the particular buckyball experiment are a different experiment than without the photons, i.e. different boundary conditions and therefore the wavefunctions will be different. This does not mean that they have decohered in the sense of becoming classical, but that the solution is such that the phases are cancel. An analogous example is penetration through a barrier, where the solutions are such that probabilities of detection become exponential within the barrier. Nothing classical about it. 

Why is it then that the forces holding the molecule together (I guess the electron sharing or covalent bonds?), and the forces holding the atoms together (I'm guessing electromagnetic forces between the electrons and nuclei, and nuclear forces within the nuclei?) are insufficient to cause decoherence?

What is observed with the bucky balls in not decoherence per se,i.e. classical behavior, but a specific quantum mechanical behavior. The forces holding the molecules together in the buckyball are a self contained quantum mechanical system modeled  as one wavefunction.
When more than one particle is involved, as in nuclei and molecules, if the dimensions are commensurate to h_bar,  one is in the quantum mechanical regime, and the wavefunction of the whole  system has to be found, solving the quantum mechanical equations with the given boundary conditions. These solutions will be "fuzzy" because in this regime the complex functions have phases that will control the probability distributions when squared.
A good formalism  for many particles is the density matrix approach. In this formalism the phases between all the contributing states are in the off diagonal elements. Decoherence happens when the off diagonal elements are zero, which is then the classical regime. It is not the observation that introduces the classical regime, but the macroscopic dimensions in a problem with order of  avogadro number  number of particles  and distances where the Heisenberg uncertainty is always fulfilled.
Maybe this double slit experiment will clear things up for you.


Double slit single electron at a time
  The observation is the hit (x,y) of the electron on the screen. Each individual dot is the footprint of an elementary particle . The quantum mechanical "fuzziness" only appears with the accumulation of many similar points, and shows the probability distribution for the electron to go through the slit. The problem setup is " electron of momentum p scattering off two slits" and nature gives us the probability distribution with the accumulation. So the observation of each individual electron did not destroy the phases inherent in the quantum mechanical solution of the problem.

Decoherence of a wave function is not induced by observation/measurement but by  changes in the boundary conditions or by very large numbers as described above. For changes of boundary conditions and loss of double slit signal read the next paragraph in the link.
