You said you had questions and then you wrote what looked like 3 statements to me. But I'll treat it like you were asking questions about the 3 things.
The Whales are exploiting the system and eating up all the reward cards that they will then sell back to you. This was done to protect smaller collections.
Question potions used to cost $2 for both credits and DEC. When DEC skyrocketed you could take advantage of the system. 2000 DEC used to buy 1 potion. Now you can get a potion for 70 DEC. This was used as an exploit... Sell the DEC turn to credits and now you have 7x the buying power for credits.
You can buy cards with credits, you can rent cards with credits, you can buy skins with credits and in a few weeks you can buy card packs with credits.