Prior remnant
Mercury
A core with almost no coat left
Most of the planet is core. It has shrunk. Ice hides in polar dark. The last ash of a finished world, or a stripped first-generation body.
What missions and papers show
MESSENGER mapped old dark terrain, volcanic plains, and thousands of shortening landforms. Radial contraction of order five to seven kilometres — cooling of the body Mercury already is, not a Venus-sized world shrinking into this one. Mean crust about 35 km over a giant metal interior. Low-reflectance material is consistent with a carbon-bearing ancient crust.
How this site reads it
This site reads Mercury as a further remnant class after Venus: a core with almost nothing left above the rock. Last ash of a finished world, or a stripped first-generation body. That fork stays open.
What would kill this stage
If Mercury was always small and mostly iron from the start, the remnant reading comes off the table.