The apparent (longitudinal) coordinate singularity at the 90 degree latitude in spherical coordinates is an artifact of the coordinate system chosen, which is singular at the poles. A different coordinate system would eliminate the apparent discontinuity, e.g. by replacing the latitude/longitude representation with an n-vector representation.
A spherical object like the Earth, or a black hole that rotates, becomes an oblate spheroid due to it's rotation. Mapping an oblate spheroid to an equirectangular projection will result in distortion.
Gravity around a black hole is so powerful that it affects both space and time, for that reason a metric (to use your word, map projectionsprojection) is used to describe the space, and not differential geometry. In order to take gravity into account, physicists use the theory of general relativity, which is formulated in the mathematics of a non-Euclidean geometry.
Even zero sized particles can have ill defined boundaries, and many clumped together make exact analysis and estimation of the center,center; in order to provide a location, a difficult task. Because of their zero dimension many can occupy the same location, passing through each other, or createmove minutely apart creating an extended object.
A small amount of mass is much larger than a point and so is a black hole. TheyBlack holes only occupy such a small areaamount of space because they (bend) compress the area to tiny dimensions. The space surrounding such a large mass (often over 10 times the mass of our Sun) occupying such a small area is a point, the (a singularity). A black hole would not be a point sized dimension if it's gravity could be turned off, if it occupied a significantly larger area (reduced density), or it's mass (weight) where enormously reduced.