I sign Colton.
The positioning of the signal there is, frankly, horrific, with no banner repeaters and the positioning of the overhead stations - but surely your TOC would also tell you to stop as soon as possible if given a wrong route?
For me, while I can still get to where I need to be by taking either route (I sign both), the chances are that my guard only signs the mainline.
I think what you're describing is over the top cautiousness by a TOC. There's nothing wrong with the signal at Colton, you don't need to sight a green within service braking distance on the off chance you’ve been wrong routed.
At some signals a wrong route is unavoidable if offered (Colton being a good example). Any half decent employer would fully back their driver and also appreciate that the performance implications of even suggesting every train on the up fast at Colton should slow from 125 to be able to stop, just incase of a wrong route, far outweighs the safety benefit for the once in a blue moon the signaller (or, to be far more accurate, ARS) gets it wrong. Ultimately thats what it is - signaller error.
Luckily both my previous TOC employer and current employer take the common sense approach.
Back on topic, assuming Euston works the same as other locations (train gets checked to red, subsequently clears to position light with platform number), driver should have queried the signal, signaller shouldn't have routed it. 50/50 blame if the signaller was aware of the train formation. All on the driver if booked 350*2 but actually presented as 730*2 and signaller was unaware (though arguably theres some form of system error to take into account if the data didnt pass along).
Sounds like it was handled atrociously mind, as pretty much everyone has stated, you'd lock off the hanging off doors and de-train pax from the bits on the platform. Shunting with people onboard is never preferable, never mind agitated people who are now 30 mins late.