Did the Divine actually SLOW the vehicle? What was the likely result without divine intervention, and whose karma was involved here?

There was a divine intervention, but of a somewhat different source than you might expect. This was not an external insertion of divine energy, but rather a divine impulse within her to win the day through taking swift and sure action to maneuver the vehicle in the best possible way, to not having a worsening of things with a more serious collision that could have resulted in injuries and loss of the vehicle as well. This was an answer to prayers to allow the higher self to act on behalf of the prayer requests, to provide the inner knowing and sureness to act almost without thinking in a reflexive way. This is often a combination of learned skill, the muscle memory to carry out the physical motions needed, but also when the subconscious moves to the foreground because of ongoing stressful conditions. This allows the divine realm to be more actively engaged to support the person with the right instinctive thoughts and actions, and it is an equal consequence of that person’s spiritual alignment to allow the interplay to be so very effective. Being her at the wheel, any karmic underpinnings were hers, but the proximal culprit was the loss of brake fluid, and this would put any driver to an acid test, so to speak, which she passed with flying colors.