About your Arena questions, the Goblin problem is a bug. It can be annoying when one is not familiar with their actual skill levels for Arena.
If you retreat from battle, it does count as a loss. This is normal.
Yes, changing the difficulty will yield greater rewards if you win 8 battles. Do not be surprise how difficult it is for you to achieve that.
About the gems, you can open Gem Chests and Event Chests. You can also buy souls, gold, and armor from the shop. I would recommend spending your gems on Event chests.
Maybe someone else can answer your PvP questions better than I can. Only thing that matters is to win those trophies for your guild. Ranking up the leagues will earn you bonus gold when you log in daily.