The Pompey - St Malo ferry seems to sell out of bike spaces very early.
When on that route a year or so ago someone in the queue had tried to book a cabin for travelling with bikes but found it impossible but you can do it with a car, so she rang and complained and was told cabins were for car drivers: so maybe try to book on that route using the "reserved tilting seats" and see if you can get on like that. We did that - but cos it was cheaper.
Great write-up of your trip. Am admiring your choice of cycling shirt. Very stylish.....