1. The resizing process isn't simply copying the files. The most important is to keep the file system tables and metadata up to date. This is done by the various specific software tools used by GParted (gnu parted, ntfsresize etc).
Furthermore, working on the same hard drive is much slower than copying data from a hard drive to another. Heads move all the time between various places, from data to tables, to new table place, to new data place, and so on. Something similar happens to the defragmenting process too.
GParted tries to use the most efficient block size for the various operations.
Due to the large data amounts, it wouldn't be possible to everything read on RAM and then send to the hard drive in order.
The time indication and prediction isn't always fiable. Unfortunately, this comes from the third party tools. The process isn't "linear' in the time.
2-3. Resizing a formatted partition that doesn't contain data is of course very fast, usually a few seconds.
Resizing by changing the end of the partition only, is usually much faster than modifying the start of the partition.
Changing the partition start point relocates everything.
By cancelling the resize process it self very often damages the file system. Cancelling the initial simulation process doesn't make any problem.
It is always possible to reformat or recreate a broken partition and restore the data from a backup (this is valid for data partitions; for system partitions it isn't so simple to just restore content). There is nothing harmful for the hardware. Perhaps it is even better for the hardware, because a hard drive running at full capacity for long is quite stressing from the mechanical and the thermal aspect, sure.
There was reported a strange case lately: a hard drive that didn't allow to be repartitioned by various s/w. After deleting the partitions, the partition table, overwriting the drive with zero, the old partition table reappears! We suspect that there was a "firmware lock" by the computer manufacturer and seller. This wasn't specific to GParted.
Finally, there is a subtle point to take into account, concerning the latest hard drive generations, like SSD drives and the 4096-byte/sector rotating drives (it seems that the new Western Digital drives, named "Advanced format" drives, belong to this family). These drives perform better with their partitions aligned to the 4KiB segment boundaries and preferably to the MiB boundary (that's why late GParted versions default to that). We often want to realign the hard drive's partitions by restoring the content in new aligned partitions. Imaging software isn't good for this, because the image is conform to the previous cylinder alignment.
*** It is highly recommended to backup any important files before doing resize/move operations. ***