You never mention the Minimum Free Space setting which can be relevant in selecting a drive
As to the questions:
Difficult to answer definitively as it can depend on many factors (such as the disk controller), and what Unraid currently has in RAM at the time in terms of directory information. In ideal conditions only the required drive is spun up.
It probably depends on the client. You will definitely get an out-of-space error for the file that does not fit - whether the others then copy will depend on the client (but they should then go to another drive as long as the share settings allow another drive to be selected.
Mover works at the individual file level not the folder level. At the point it selects a drive a file size of 0 is assumed (and thus the importance of the Minimum Free space setting).
If a file already exists then Unraid will always attempt to overwrite it in situ so you get an out-of-space error. Not quite sure what you meant by Unraid balancing out, but it never moves a file from one array drive to another.