Can mold spread to unopened butter

Assuming that there are no apparent problems with it (either from storage method or upon opening and inspection), I am one to agree with StillTasty in this case and say that you would not likely encounter any problems making barbecue sauce. They say twelve months, though.

For the sauce though, not only will the long, slow boiling help; but the ketchup is likely chock full of high-fructose corn syrup (which acts as a preservative), and you will be suspending more sugar (again, a preservative) in it.

Per NC State's Extension's article on pickles and sauerkraut (some emphasis added):

Pickles or sauerkraut mold during fermentation.

Answer:

Unsafe—microorganisms are growing improperly.

Possible reasons

• Fermentation temperature was above 75°F.
• Too much salt was used, not allowing adequate lactic acid production.
• The cloth on top of the kraut was not kept clean during fermentation (may need to be replaced after skimming).

Per Penn State Extension's Saurkraut Guide (emphasis added):

Do not taste it if you see mold on the surface, feel a slimy texture, or smell a bad odor.

Their PDF on Saurkraut says:

To avoid surface mold growth, keep the cabbage submerged at all times by covering it with a plate just small enough to ft inside the fermentation container or with two or three clean quart jars filled with water. An acceptable alternative is to fill a large, sealed, food-grade plastic bag containing 4½ tablespoons of salt and 3 quarts of water.

Again, no indication of acceptable types of mold.

I cannot endorse the existing answers which indicate that mold is an acceptable part of the saurkraut making process, unless credible science based evidence is presented, from reputable sources such as University Extension Centers or, better yet, peer reviewed journal sources.