It says that you can only deposit/ withdraw keys in the inventory section. If I was to put 6 keys into my inventory and purchase a game gift for 5.2 keys, how could I withdraw .8 keys if it only allows me to withdraw whole keys?

You can't. If you are using Bitcoin, you could purchase the missing 0.2 credits and then withdraw the whole key, otherwise i don't think you got many other options, except maybe buying refined metal, etc.

No clue, you gotta use market search ans see what sellers are asking for it or you can place own bid order with the price that works for you and wait for someone to fill it.