OP, you can only do your best..
..but IMO temper your expectations!
Every contemporaneous photo, both your sister's and the CEO's, shows the location of the sign clearly and once this is known then the driver's duty is to read. IMO, an adjudicator would not find otherwise.
And the tree concerned isn't a street tree, it stands on private property, so be prepared for a 'not the council we don't keep records of treatment of trees on private property.