When exposing the mainFilestore folder as a read-only share, it is important that it has the correct permissions assigned.
Use this command to grand correct rights on mainFilestore
icacls "D:\Elvis Data\mainFilestore" /grant:r "Everyone:R" /T
icacls "D:\Elvis Data\mainFilestore" /grant:r "COMPUTERNAME\Guest:R" /T
Share your mainFileStore as ElvisDrive (any other name will do, keep away from special characters)
Change the rights on the folder tempFilestore to the rights you need to have on the files in the mainFilestore. Because of the way Windows 2008 and Elvis works permissions on files moved are assigned to the file in the target directory no matter what you have set on the target directory. When you adjust rights on tempFilestore you are fine.