It is not Embraer procedure to select flaps 5 during deceleration. It's part of the evacuation checklist only.
Hydraulics are not an issue, the flaps run on AC power. Not much of a difference, though. Once you shut down the engines, they won't move anymore.
If you look at the Embraer QRH procedure (which is in the report right below the Flybe vital items) it only requires to set the lever to 5 and does not ask to wait or check for actual deployment. (In general Embraer manufacturer's procedures cannot be lauded for consistency with normal and abnormal checklists using different naming philosophies and items used every flight referred to by ridiculously long wordings while at the same time not requiring to check a system state but rather if the lever is in the correct position.)
Regarding the position of the trailing edge flaps, there is no difference between flaps 3, 4 and 5 as only the slats move between 3 and 4/5.
It would of course make sense to mark overwing escape routes in day-glow or similar, but airlines seem to loathe painting stuff on planes that reminds passengers of emergencies (like marking exit outlines in light grey which is much less conspicuous to emergency responders than a red frame).