What is the meaning of "being safe from Allah's devices/schemes (makr)"? And does not being safe from Allah's devices/schemes contradict having a good opinion of Allah?

Linguistically, security means not expecting any harm and the disappearance of fear. The cautioned against is the servant's heedlessness of God and his feeling of security from God's punishment. The believer combines fear and hope in his worship, as in the Almighty’s saying: "Is one who is devoutly obedient during periods of the night, prostrating and standing, fearing the Hereafter and hoping for the mercy of his Lord, [like one who disbelieves]?" {Az-Zumar: 9}.

Fear and hope do not contradict each other; believers fear their Lord and hope for His mercy. Moderation between fear and hope is necessary to avoid arrogance or despair. In good health, it is preferable to lean towards fear, while at the time of death, hope and good presumption of God should prevail.

Acceptable good presumption of God is accompanied by performing acts of obedience and avoiding sins. However, good presumption while neglecting duties and committing prohibitions is blameworthy and is considered feeling secure from God’s devise.