Yes, you can plug your hdd there forever. Yes, it will heat up
as, your hdd runs, and when it runs motion produces heat.
Durability, hmm, let's put it in this way: if your hdd in your
computer is able to work for 9hrs/10hrs and last for like 3-4 years
or more. your external hdd should be able to do the same. Just put
it in a spot where there might be a fan blowing on it.
It depends on how you configure your ext hdd to backup. (you
will have to refer to the software given for backup) Yes, this
includes hidden viruses/malware