1) Each pair of floor-level doors are meant to be covered during boarding. Like you mention, a big reason is in case there is a need to initiate an evacuation or a rapid disembarkation. Also, even at stations where boarding passes aren't checked, we check the ID's of ground service staff boarding the aircraft.

2) We haven't seen the route moves for the summer yet but it's most likely that it'll move to Mixed Fleet. Not just because of being Percy Pig trained but because of the Worldwide Scheduling Agreement for operating shorthaul routes as opposed to Mixed Fleet's Mixed flying Scheduling Framework. Not sure about their crew complement but assume a full complement is always a CSD and CSL, regardless of duty type.

3) "Senior Cabin Crew" when referring to ICC's are the equivalent of their line managers. There is also a base manager. They don't operate a senior role onboard but have additional responsibilities at their base. Silver ties usually aren't worn in any publicity or advertising.