Being in Minnesota, I'd imagine that your trapping conditions might be similar to ND in regards to weather, soil, etc. A lot depends on when you are trapping. In the dead of winter you aren't going to be able to get a berkshire in to the ground in most spots so length is kind of irrelevant, unless you drill a pilot hole (which is a lot of abuse on equipment and you need to power it in sub zero weather).
However, early in the year before the ground turns to concrete you will likely need close to those 20 inches as your testing has revealed. But later in the year, you could easily use only a foot where the ground is froze up solid. There are times I might only get a wolf fang in as little as 6 inches and there's not a chance that anchor is ever coming out until the ground thaws, but make sure to watch out for a warm spell. It would take a whole lot of energy and wasted time to sit and beat on my driver until I got the thing in any further and it wouldn't make a difference.
As far as dipping and dying the anchor. I would highly recommend that you not dye the cable as I believe it will cause the cable to dry out faster and result in cracking, which you don't want. There is some oil in the cable, and that is your friend. Could you dip it in wax? Sure, but not necessary.