Was the mercy seat covered and caked with blood?
No. The high priest did not sprinkle gobs of blood on the mercy seat. He dipped his finger in the basin and touched the golden mercy seat, presenting the blood without covering it in layers of dried blood. The text describes a precise, minimal application, not a surface caked with blood.
Les explains that the high priest came behind the veil once a year and sprinkled the blood of the bullock and then the goat upon the mercy seat. Many people think that seat must have been filthy with dried blood, but Les says they did not sprinkle gobs of blood around. Instead, the priest took his finger, dipped it in the basin, and just touched it on the golden mercy seat so there were not gobs of blood upon the gold. Les notes he has had that question come up more than once, and the priest simply used his finger to present the blood.