lazerdye

"48.4 sy 51.5 wa" on 3rd line partially answers your second and fourth questions - SATA interface on A20 is slow and inefficient, so swapping combined with running torrent client results in this - CPU is processing SATA requests and waiting for I/O operations (and if you have mechanical HDD, seek time when multiple programs access different files also affects this).
 
So in your case using swap on SATA is bad idea, using no swap (if possible), zram or swap on SD card may be better.

If you are in a need you can use this updated script:
https://github.com/igorpecovnik/lib/blob/master/scripts/nand-sata-install/usr/lib/nand-sata-install/nand-sata-install.sh

I think we haven't tested SATA installer on XU4 ... so it's possible that something important is missing from kernel. I can check and rebuild kernel in few days.

I had a similar problem with an olimex a20 board, ended up being the overclock that armbian applies.