Zammo

If the drive's failing but stiil currently readable, then you could just tell DrivePool to Remove it and it will evacuate the data file by file to the rest of the drives in your pool (usually the one(s) with the most free space).
If you're wanting to manually move the content as you describe (e.g. because you're using SnapRaid or because you want it done ASAP and don't have duplication), then as long as you turn off any Balancing and Placement, and stop the StableBit DrivePool service you can safely move* content between the poolparts so long as the internal structure is kept the same (e.g. don't change "pathX\fileY" in "Drive1:\PoolPart1342343\pathX\fileY" to "Drive2:\PoolPart324343\pathX\fileY"). Once you're done you can then restart the service, Remove the drive you manually emptied, and then turn back on any Balancing and Placement.
*Note: for a failing disk with no duplication, instead of doing a "direct" move, I would copy the content to the destination and verify the copies before I deleted any content from the source. This minimises writes to the suspect disk while I'm getting data from it.
"Also, the drives are SnapRaided, would it be safer to copy everything off and hope everything is in tact data wise, or restore from the parity drive ?"
See the SnapRaid FAQ for the "100% safe way to proceed": https://www.snapraid.it/faq#move

Yes, if you click Remove on the dying drive then it will automatically move everything (in the pool) that's on that drive to the other drives (in the pool). You may or may not wish to choose the option of "Force damaged drive removal (unreadable files will be left on the disk)." This will help if the drive has damaged files that can't be read.
However, since you are using SnapRaid, you should instead be incorporating its instructions to safely transfer the files without messing up your parity (Disclaimer: I'm not great with SnapRaid, please double-check this):
(turn off DrivePool balancing/placement if you're using it - which you probably aren't using if you're using SnapRaid) (stop DrivePool service) copy files from good Drive2 to good Drive3 check with diff, if good (identified as copies) proceed with a sync and continue to next step delete files on good Drive2 run a sync copy files from failing Drive1 to good Drive2 check with diff, if good (identified as copies) proceed with a sync and continue to next step delete files on failing Drive1 run a sync might as well take the failing Drive1 out of the machine at this point - run a secure erase on it if you need/want to - but remember to follow the SnapRaid procedure for removing a data disk from the array (start DrivePool service) (remove the "missing" Drive1) (might need to do a Manage Pool -> Re-measure if DrivePool doesn't do it automatically) (turn on DrivePool balancing/placement if you're using it - which you probably aren't using if you're using SnapRaid) You can still use Remove on a drive containing unduplicated files; it will still move them to another drive. DrivePool duplication, if enabled, is like RAID1 in that it ensures that duplicated files exist on two or more drives (so that if one drive dies unexpectedly you haven't lost any of the duplicated files that were on it as they will also be on at least one other drive). 2x duplication does use 2x the space of course, and so on. You can turn it on for the entire pool or customise it for specific folders.

No.  There is some very hidden data that doesn't copy over, really.  But if the new drive is added to the pool, you could copy the contents of the PoolPart folder on the old drive to the PoolPart folder on the new drive.   Just make sure you remeasure after it's done. 

I think DrivePool will ignore folders that it hasn't created itself. Never tried this actually.
I'm not using any backup solution myself, just JBOD hurdled together in DrivePool as a single drive, and I've had my fair share of disks that have failed (FUBAR cables, for the most part), and also disks that were full (Like 0 bytes left) which have led to having to move data, and I found that it was working to move data outside of the PoolPart-folder made by DrivePool, and then to/from source/destination.
But, remember to pay heed to never move data from one PoolPart-folder to another since DrivePool tends to pull a David Copperfield on you and make stuff disappear, like magic.
I have yet to try out recovery software to see if removed data can be restored, come to think of it. Not that it has anything to do with this, but it should work I think.
I find it very simple to work with the folder structure (PoolPart) since anything within it, you can just move up one level and it's taken out of the pool, then you can merge that data with another folder on another disk or whatever then you can just put it back and et voila - It's back in the pool again.