 Here is a Python script that performs the tasks you've described using the pyscf, pywannier90, and mcu libraries. Please make sure to install these libraries before running the script.

```python
import pyscf
import pywannier90 as pw
import numpy as np
from ase import Atoms
from mcubed import mc, plot

# Define the unit cell
atoms = Atoms('Si',
              numbers=[1, 1, 1, 1, 1, 1, 1, 1],
              positions=[(0, 0, 0), (0.5, 0.5, 0), (0.5, 0, 0.5), (0, 0.5, 0.5),
                         (0.5, 0.5, 0.5), (0.25, 0.25, 0.25), (0.75, 0.75, 0.75), (0.25, 0.75, 0.25)],
              cell=(5.43, 5.43, 5.43))

# Perform PBE calculation
mf = pyscf.gto.Mole(atoms=atoms, spin=0, basis='6-31g')
mf.scf()

# Save and load the kks object
kpts = np.array([[0, 0, 0], [0.5, 0.5, 0], [0.5, 0, 0.5], [0, 0.5, 0.5]])
kpts = mf.make_kpts(kpts)
kpts_grid = mf.make_kpts([[0, 0, 0], [1, 1, 1]])
kpts_grid = np.array(kpts_grid)
kpts_grid = kpts_grid.reshape((2, 2, 2, len(kpts[0])))
kpts = np.array(kpts)
kpts = kpts.reshape((4, len(kpts[0])))

mf.kpts = kpts
mf.kpts_grid = kpts_grid
mf.save('si_pbe.hdf5')

# Load the kks object
mf = pyscf.io.read('si_pbe.hdf5')

# Construct MLWFs
pw.wannier90(mf, kpts, kpts_grid, nwfr=8, wigner_sym=True, wannier_centers=[[0, 0, 0], [0.5, 0.5, 0.5]],
             outdir='si_wannier90')

# Export the MLWFs in xsf format for plotting
pw.xsf('si_wannier90/wannier.xsf')

# Export certain matrices
pw.wannier90(mf, kpts, kpts_grid, nwfr=8, wigner_sym=True, wannier_centers=[[0, 0, 0], [0.5, 0.5, 0.5]],
             outdir='si_wannier90', wannier_matrices=True)

# Run a wannier90 using these
pw.wannier90(mf, kpts, kpts_grid, nwfr=8, wigner_sym=True, wannier_centers=[[0, 0, 0], [0.5, 0.5, 0.5]],
             outdir='si_wannier90', wannier_matrices=True, wannier90='wannier90')

# Interpolate the Fock or band structure using the Slater-Koster scheme
pw.wannier90(mf, kpts, kpts_grid, nwfr=8, wigner_sym=True, wannier_centers=[[0, 0, 0], [0.5, 0.5, 0.5]],
             outdir='si_wannier90', wannier_matrices=True, wannier90='wannier90', interpolate=True)

# Print the difference in the eigenvalues interpolated by scf.get_bands function and by pywannier90
e_scf = mf.get_eigenvalues()
e_pw = np.loadtxt('si_wannier90/wannier.eig')
print(np.max(np.abs(e_scf - e_pw)))

# Plot the band structure using mcu
e_pw = e_pw.reshape((8, len(e_pw)))
mc.set_plot_style('matplotlib')
plot(e_pw, labels=['Band %i' % i for i in range(1, 9)], xlabel='Energy (eV)', ylabel='')
```

This script defines a silicon unit cell, performs a PBE calculation, saves and loads the kks object, constructs MLWFs, exports the MLWFs in xsf format, exports certain matrices, runs a wannier90 using these, interpolates the Fock or band structure using the Slater-Koster scheme, prints the difference in the eigenvalues interpolated by scf.get_bands function and by pywannier90, and plots the band structure using mcu.