I would not think of God putting a "mark" on anyone as it seemed to me that you were talking about some sort of a physical mark like a tattoo. As for the phrase "seal of God," my thinking is twofold: First, Ephesians 1:13 - "And you also were included in Christ when you heard the message of truth, the gospel of your salvation. When you believed, you were marked in him with a seal, the promised Holy Spirit," Second, the manifest character of the "seal" is found in 1 Corinthians 13:1 - "If I speak in the tongues of men or of angels, but do not have love, I am only a resounding gong or a clanging cymbal."

As for question #2, here is this excerpt from Romans (8:37-39) - "No, in all these things we are more than conquerors through him who loved us. For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord."