So, if the Earth is not an ellipsoid then how can 1˚ of longitude equal approximately 60 nautical miles and yet, at a latitude (North or South) of 90˚ it is equal to 0 nautical miles, and scales up and down in accordance with the curvature of the ellipsoid (allowing for delta major and minor differences), so that anyone using WGS84 in a ship, having to convert that into UTM Eastings and Northings, is able to, with pinpoint accuracy in a well calibrated system repeatedly find exactly the same spot.

Basically, if the Earth was flat then how come non-Euclidean (spheroid) geometry can be used in the real world with demonstrable, repeatable results and can easily be manually calculated and converted between co-ordinate systems?

If the Earth was flat then all geometric descriptions of the relative positions would conform to Euclidean geometry with only local geoid corrections to be applied (like mountains and valleys and stuff).

If the Earth was flat then gnomic charts and great circle navigation wouldn't work (at all) and yet ocean going vessels use them day in and day out for long transits and always get where they are going with no significant surprises.