In answer to the first question, generaly the cabin is pressurized to -200ft before take off to ease the transition to flight and make it more comfortable for the passengers. So initialy the cabin will show a brief rate of decent.
As for ditching at sea and sea dump valves...???????? I would have thought the cat flap would have opened and shipped water due to the excessive external pressure.
In 737 the air conditioning packs provide air cooled at the temp of the coolest zone requested in the aircraft. The mixer valves then warm the air to the temp required by the other zones.
A thermal relief valve relieves excess pressure due to an increase in temperature.
As for the blower, to long ago to remember sorry....