Why do aromatic compounds have an upfield shift upon coordination to metals?
Chemical shift data dug up for a number of iron compounds in the website of Hans Reich reveals that the shift is not unique to aromatic compounds, but rather appears due to Fe(2+):



The base shift for a vinyl proton (C=C-H) is 5.25 ppm. 

The following is a series of spectra from magritek, illustrating the effect of Fe(2+) complexation on the shift of the cyclopentadienyl ligand.



Sandwich compounds based on magnesium do not show such large upfield shifts:

 

This seems to eliminate your conjecture 3 as an explanation, since cyclopentadienyl is aromatic and addition of Fe(2+) causes an upfield shift, whereas Mg(2+) does not cause such a large one. 

Now Fe(2+) complexes may be paramagnetic or diamagnetic depending on the the ligand (low/high field). Paramagnetic metals have unpaired electrons that can induce strong dipolar magnetic fields. In porphin, for instance, paramagnetic Fe(3+) results in strong shielding in the aromatic ligand, with one proton shifted upfield ~20 ppm. A diamagnetic Zn(porphin) complex does not show such strong shielding. Ferrocene is diamagnetic.

The quick and sloppy answer therefore appears to be that the shift observed in ferrocene is mainly due to Fe(2+), even though the compound as a whole is diamagnetic. 

Regarding the bonding arrangement, the wikipedia has this to say:


  In terms of bonding, the iron center in ferrocene is usually assigned
  to the +2 oxidation state, consistent with measurements using
  Mössbauer spectroscopy. Each cyclopentadienyl (Cp) ring is then
  allocated a single negative charge, bringing the number of π-electrons
  on each ring to six, and thus making them aromatic. These twelve
  electrons (six from each ring) are then shared with the metal via
  covalent bonding. When combined with the six d-electrons on Fe2+, the
  complex attains an 18-electron configuration.


To add a little more detail, Mulay and Fox hypothesize on the nature of metal-ligand bonding in ferrocene based on the magnetic anisotropy:


  The anisotropy between K3 and the average of K1 and K2 was found to be
  $\pu{49.5×10—6 cgs units}$, and is evidence in favor of a single dπ—pπ bond between each of the rings and the iron atom. Pauling's "resonance
  structures" also seem plausible; the studies do not support Fischer's
  "complete π‐electron donation" structure. Theoretical calculations
  based on Langevin's formula for diamagnetism show a close agreement
  with the experimental value.


A more detailed account of bonding in ferrocene can be found in Miessler and Tarr's textbook.

[6]: Magnetic Anisotropy of Ferrocene and Chemical Bonding, J. Chem. Phys. 38, 760 (1963); https://doi.org/10.1063/1.1733734, L. N. Mulay and S.N.D. Mary Eleanor Fox