Fe³⁺/Fe²⁺ is a very electropositive oxidator, which means that there are few steps in the electron transport to oxygen. Also, the relatively poor energetic yield from this oxidation coupled with the large energetic demands of the Calvin cycle means that A. Ferrooxidans must oxidize large amounts of Fe²⁺ to produce a small amount of cell material.

Fe2+ is oxidized to Fe3+ by an outer membrane cytochrome c that transfer electrons into the periplasm where rusticanin is the electron acceptor. Rusticyanin reduces periplasmic cytochrome c, which transfers  electrons to cytochrome aafont-weight3, which reduces O2 to H2O.

Yes, ferrous iron can be oxidized under anoxic conditions by chemolithotrophs. Fe²⁺ is either an electron donor in energy metabolism and/or a reductant for CO₂ fixation. The reduction potential is more electronegative at neutral pH than at acidic pH.