Tomnick,
It is the driver's responsibility to know exactly where he is going and while in the case of having had a single yellow and that the subsidiary signal ahead may be at red, due to a technical problem, he should still see that the signal is 'off', even if the line speed means that he can't stop at it. He would get away with it on that technicality, as it would be a signalling irregularity.
I agree that this subsidiary signal is mainly acting as a 'proving' signal but it is still part of the route set up. If it isn't 'off' how would the driver know that the route was correctly set up and all points locked! It doesn't happen very often but I have known of it.
Likewise, if a driver comes up to a junction divergence signal and the signalman has set the wrong route it is still the driver's responsibility to notice the fact and attempt to stop at the signal and report it. If he can't stop he would then pull up as quickly as possible and get onto the signalman immediately by whatever means available.
He may be told to continue at caution and obey all other signals or to reverse back behind the signal and wait for the correct route to be cleared for him.
In the case of changing ends the driver would know which signal he required and would stop over it and then change ends. This could be out on the track or on a platform. That is considered a shunt move but inside station limits you can, in some cases, have a single yellow to run, say, into a siding, or bay platform where trains normally depart from. But mostly you would have the subsidiary signal to make the initial movement.
Some large stations have signalling especially designed for that layout only and you can find movements from stop boards, subsidiary and main aspect signalling that would not be allowed elsewhere. Birmingham New Street and Bristol TM are two examples. This comes under local agreement and route knowledge and is covered by the Regulations.