(1)
very easy to calculate. for 1000ft2 of 25mm thk 1:3 cement sand screed = 92.93m2 (convert to meter square) = 2.32m3 (convert to m3)

cement density is 1600kg/m3
sand density is 1700kg/m3

2.32m3 x 0.25 x 1600kg = 928kg
928kg / 50kg = 18.56 or 19 bags allow some wastage la 21 bags.

Why 2.32m3 x 0.25? where is o.25 come from?
If sand is 25kg per bag means:
- 2.32m3 x 0.25 x 1700kg = 986kg
- 986kg /25kg =39.44
- 1:3 = 39.44 x 3 = 118.32 bag??

(2)
there is no layer of cement. it's concrete which is the mixture of cement, sand and aggregate. the ratio is 1:3:6 for grade 30 or grade 40 concrete if I still can recalled. they either use rebar or wire mesh. 2 layer usually, top 1 bottom 1.

let say like im doing renovation at second floor and i need to hack the floor for recessed junction which is 3" depth
so im now at the concrete level, so after concrete layer how depth to reach the 1st layer of wire mesh?
will it be:
-------------- (2" of concrete)
xxxxxxxxxxx(wire mesh)
--------------(2" of concrete)
xxxxxxxxxxx(wire mesh)
---------------(2 " of concrete)

so total up will be 6" slab?

(3)
depend on design. footing alone usually enough. however some case they require tie beam connecting all the footing / pile cap.

can it be notice by just calculate the column size and column to column distance and height of building?

(4)
shear wall required to resist the lateral load especially on high rise building, the wind load is increasing as the building is higher. therefore shear wall is require. they will positioning the shear wall at the center of building. if the building is big, then they will position it center of each half of the building. staircase and lift core is the place they choose to form the shear core by constructing the shear wall.

so possible to erect a RC staircase structure with only column? noneed shear wall the? the 4' x 4' landing will be damn heave!

(5)
u need to drill a hole to existing structure and plant a rebar as dowel bar. then new beam rebar will lap to that dowel bar. usually engineer will recommend how long the depth require to embed the dowel bar and what type of epoxy glue require. these are the thing u not suppose to propose urself instead get their approval to use as that would be the integrity of structure.