Guild Wars 2

Well i just bought the dagger using the arah shards and i was wondering if you could replace the sigil on it

So I bought two superior runes of thief and when i equipped them onto my armor it says its 8/6. So did I only need to buy 1 rune? If someone can please explain

Ive ran arah explorable for awhile and recieved the shards of zhaitan but i dont know what they’re for

So I wanted to buy the medium armor skin, but can I only put it on one piece of gear? So if I eventually find a better piece and equip it, I wont have the skin on that specific piece?