The translation routine for gdt 3.10 (mercator grids) does not check all termplate parameters.
Specifically, it doesn't check the scanningMode, nor any of the following parameters at the end of the template : "Orientation of the grid", "longitudinal direction grid length", "latitudinal direction grid length"
(I haven't checked what the actual key names of those in ecCodes would be)

For comparison, the translation routine for gdt 3.12 (transverse mercator grids) does check the scanningMode parameter, and errors if contains unsupported settings.