1.  No.  There is too little chance of it being useful.  Just replace a pred fail drive.  Pred fail is still functional - and online with the array - but there is a 99% chance that the drive hardware is faulty.  When you replace a pred fail drive, ALWAYS force it offline before removing it.  Even if you choose not to purchase a drive directly from Dell (I don't blame you), you should make sure that the drive is certified for use on the controller; don't get the generic/retail version of a certified drive. 2.  Again, always force a pred fail drive offline before removing it.  Never power down a server to introduce a disk to an array; if it is hot-swap, swap it "hot". 3.  There is no harm in inserting the disk (hot!).  You will likely need to clear its "foreign" configuration and re-assign it as a hot-spare.  I would run diagnostics on the drive first though, before assigning as a hot-spare.  Hot-spares do not actually interact with the array in any way; they only sit "at the ready" in case a disk fails, then it rebuilds in its place automatically. 4.  You cannot image each disk individually (I mean, you can, but it is only useful in special data recovery scenarios); a backup of the ARRAY is the best way to go.  To backup software, the ARRAY will appear as an individual "disk" and will not know or care how many physical disks make up that virtual disk.  Restoring your backups would include configuring RAID first, then restoring to the virtual disk it creates. Let us know if you have further questions :) 
I've inherited the role of maintaining a set of four Dell PowerEdge R510 servers.  All are running PERC 6/i Integrated RAID controller.  Two of the servers are running 2 disk RAID 1 configurations and two of the servers are running 4 disk RAID 5 configurations.  The two RAID 5 servers also have a single extra global hot spare disk, so 5 disks total in those two servers.  I've been doing some research over the past 24 hours and I think I have a decent handle on how to maintain and work with the RAID arrays.  I will ask some questions here to make sure I'm 100% before going forth.  Thank you for the help in advance, and I apologize if this has been covered previously.   If you could point me in the direction of some good procedures and technical documentation that addresses how to properly manage various RAID arrays on Dell PowerEdge servers that would be great.  Again, thanks for your help and I apologize if these are very basic.  
RAID 1 and RAID 5 Issue Questions - R510 PERC 6/i