1. If the diaphragms are rigid, you should use FLOOR DIAPHRAGM. Floor diaphragm automatically does what a master slave does including figuring out an appropriate master node for each floor. It can also account for the in-plane torsion due to eccentricity between CM and CR.
2. If you are carrying out a static analysis and have calculated the vertical earthquake load, you can directly apply it as a static load on the member ( nodal load can be used for that ). If you are carrying out a dynamic analysis, you need to calculate the total mass that is capable of vibrating and assign these in ALL 3 directions for the modal calculations. Generally for long members, splitting the member into multiple segments and assigning the loads ( to be used for mass computation ) to the various nodes, would result in a better mass modeling.
3. a. when using plates you should use a mesh as opposed to using a single plate element to ensure that the shear wall is properly connected to the adjacent beams and columns. The beams and columns should be broken up such that these share common nodes with the plate mesh. That will ensure proper load distribution. Surfaces are split internally by STAAD.Pro into a mesh of plates but for that too the onus is on the user to split the beams and columns such that connectivity is established once the surfaces are internally split.
b. you may use dummy beam elements at the periphery to generate the floor loads. Assign a material with low E value and zero density to these members so that these do not contribute towards any stiffness or additional load.
c. If you are designing the shear wall by hand, you my use plates. You may take a look at the following wiki
