You will need to cut into the brickwork and install them to comply with local building regulations.
Yes vents can be in the floor instead of the wall.
Return air vents.........
Cold air return vents are the ones on the wall
The vents outside the house are typically located on the roof or on the sides of the house near the ground.
House vents are typically located both inside and outside the house. Inside the house, vents are usually found in ceilings, walls, or floors to allow air to circulate. Outside the house, vents are often located on the roof or walls to allow for proper ventilation.
No sunlight reaches that depths, although if you go far enough down there're hydrothermal vents which in return heat things up.
No. Doors will suffice.
In HVAC systems, the air typically flows from the supply vents to the return vents.
Gable vents go on the ends of the house where the peak is. At the top point of the triangle.
Hydrothermal vents can get very tall. Many of them are as tall as a three story house.
In my opinion where the supply vents are located is not as important as where the return air vents are located. the supply vents should be located towards the outer walls if possible. the return air vents should be located towards the center of the dwelling if possible. every effort should be made to get the return air vents as high as possible on the wall , if not in the ceiling.Remember you are trying to get the warm air back to the a-coil so it can be pumped outside through the refrigeration lines to the condensing unit. if the ductwork is used for heating also you should put some return air vents down low to get the cool air to the furnace. This is my personal opinion and may not be everyones.
Return air vents in each room (none in bath or restrooms) sized for the amount of cfm needed for that specific room are best but if this is not possible then pick a central location and return vent should be sized for total cfm required of house.