As far as I’m aware there’s no requirement to use the tachograph mode switch at-all if you’re using log books on domestic regulations, I don’t see why the DVSA would be interested in seeing anything other than the log book for periods that the driver is on domestic regulations, I suppose it’s feasible that they may compare what the tachographs show to what the log books show but that could really only be driving time time.
Having said that I don’t see being on domestic regulations as a reason for drivers not to have proper breaks, after-all a tired or fatigued driver is not a safe driver.