I suppose it depends how you copied /home – I typically used a tar command to compress /home contents to an archive file; complete with file/dirctory permissions (this is important).
I once had the command saved as a script for convenience, but no longer have it. You could likely find a workable procedure using ‘linux tar backup home directory’ or a similar search term.
It could; it’s the same principle; however, it’s probably better to restore it from an archive file as described, rather than simply copying it from one location to another.
This is where fstab is useful. You would add the UUID and the ‘/home’ mount point as an entry in fstab. Have a look at your own fstab file for an example of the syntax:
cat /etc/fstab
After partitions are moved, created, etc, you can use:
sudo blkid
… from within a manjaro-chroot environment, to find the appropriate UUIDs.
Yes, from that environment you are in the installed system as the root user – everything you do affects that system – careful, there be dragons there!
There’s no avoiding that, sorry. 
No, that’s not related.
I suggest you compile all the information you need and create a procedure to follow in a notebook, or similar; have everything you need laid out, no matter how trivial it might seem.
And more importantly, ask someone here for clarification of anything you’re not sure about, before committing.