The crew were OK to use DME near the Mull if so agreed in Special VFR.
This flight was a classic for SVFR: because of the icing limitation, IMC was not on and the difficult part (for VFR) was localized (at the Mull) – conditions so prevalent that a routine SVFR practice would have been expected.
Eh? SVFR is a tool to allow visual flight in airspace which would normally not allow it in the prevailing conditions. The visual references for VFR flight would still have to be met, and in any case the Mull is not in such airspace.
a straight line from waypoint change at the speed the a/c had been holding for the flight up to that point gets you there
And what speed had the a/c been 'holding for the flight' then?