Great suggestions, and great post, Eric!
Let me add a few minor points:
Most modern leather sheaths are tanned quickly, so they retain quite a bit of the acids or chromium used in the process. The moisture condensed in the sheath solubilises this corrosive stuff. I don't know if treating the sheath with wax or other water sealant could decrease this leaching, but the condensation would still occur.
A frequently overlooked way the condensation might happen is when an otherwise dry knife or axe is brought in from the cold into a warmer place and left in the sheath.
For quick, non abrasive rust removal sticking the blade a few times into an onion helps, but I never tried it for the purpose of removing stains, which are different type of oxids than the red rust. Onion might even enhance patina formation, I don't know.
The patina, i.e. stain is not hazardous. Acidic food would dissolve iron both from a shiny and a stained carbon steel blade, so both can impart metallic taste to some foods. Are patinated surfaces more prone to impart metallic taste? I don't know.
I have read on Bladeforums, that here in the USA carbon steel butcher knives are not used in large meat processing plants any more.
I don't know if it is actually true, but the explanation for this was that bacteria from the butchered animals purportedly adhered better to the surface of carbon steel knives than to that of stainless steel ones.
I can see how this can be a problem, with the amount of cutting involved.
I don't think though this is an issue for a utility/camp knife, since no one uses moras to butcher animals on industrial scale. If a carbon steel knife is properly cleaned (washed in soapy water or other detergent) after cutting potentially contaminated things, it should not pose microbiological risk if used for subsequent food preparation.
For frequently used blades, olive or vegetable oil coating should work fine, however in the long term those oils can go rancid, especially if smeared over the inner surface of the sheath, or places which are difficult to clean. A safe alternative is to use pharmacy grade mineral oil, i.e. liquid paraffin. Use only a little, the more you use the more it tends to aggregate into separate drops. Adding a little bit of beeswax would create a better adhering and more permanent coating. However beeswax itself may contain varying amounts of organic acid, so if shiny mint look is the goal, better use Renaissance wax.
Remember that mineral oil softens leather and thus with time weakens the sheath, so make sure it does not rub too much onto the sheath.
, and is safe enough to drink, though you should probably save that for when your guts need a little 'loosening up'
