"Heavy Mining" stating that the second Blight cannot destroy Presence or cascade is just a reminder of the general rule that Presence destruction and cascades happen
if Blight is added, not on a per-Blight basis – it's not a special ability granted to Sweden's second Blight to make it uniquely less harmful. (In the base game, this effect was the only was the only way 2 blight could be added simultaneously, so the rule was printed on the Adversary.)
Tags: Blight, Kingdom of Sweden, Bestow the Endurance of Bedrock (Stone's Unyielding Defiance)