Hey Guyon, Great questions! So far as I have been able to find out from Sno-Seal is its good stuff, and wont soften the leather so much that its floppy. Mink oil is an old time leather preservative, but like you said it can be rather invasive (and tasty to the family dog

) not only does it over soften the leather but it also can leave behind that nasty white residue that I seem to see on half the old sheaths and holsters at gun shows.
Some other good ones I have found over the years are: Aussie Leather Conditioner by Fiebings, gooey stuff, but a little goes a long way, and it smells good too. Its base is beeswax like Sno Seal. Another Fiebings product is Leather Balm with Atom Wax, its a thin watery product that penetrates well and leaves a nice warm shine. Its what is on 90% of the sheaths I put out there. I like it because its light, doesnt leave behind a bad residue, and its open to any other treatment that the new owner sees fit, be it Aussie Conditioner, Sno Seal, etc.
I'd avoid most Neatsfoot oils as most out there have petroleum bases, theres only a few out there that are pure. The neatsfoot oil at wal mart and such are highly volital (flamable) and really eat up a sheath in short order, and they smell like Turps. :barf:
There are some good conditioners and preservatives available at good boot/shoe shops. Simply reading the can will tell the tale about them. Some are made to soften the leather for footwear but will normally state it clearly, those are to be avoided as well. The key words for weatherproofing are beeswax, and natural oils and preservatives.
As far as the darkening, they all seem to do it to a point, the deeper a treatment penetrates the darker the result. the Atom Wax doesnt darken too awful much, but it does leave a nicer finish in a nice warm shine where the Aussie conditioner will leave an almost satin finish.
I'll check the ingredients on my Aussie Conditioner and Leather Balm and get back here with the listings.

but none of them seem to harm the leather in the long run, so far all mentioned treatments are good stuff. Even the venerable saddle soap is good stuff, all be it rather messy, but not as good at repelling water as the aformentioned stuff.
Hope this helps a little. Any other questions, suggestions, comments, would be much appreciated.