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Hypothetically, based on my experience with the three healing cards, if we assume that these 3 coal cards are the same level, having coal galore is the same thing as having more coal and rigged coal. It allows you to use 1 card instead of 2 so you can reserve space for other cards. I haven't confirmed this with the developers or any expert of some sort, so it's basically my theory.

Yes, it is possible to get those cards because they are all different, yet the chances are very low. However, it would be better to have super healing alone because it's basically healing bonus and healing blocks combined into one card, allowing you to have other powerful cards to use. The reason for this is one time when I was playing I got healing bonus (level 1) which gave me 5 durability and super healing (level 1) which also gives 5 durability in addition to 100% more healing blocks. I thought that because of having two cards that both give 5 durability, I would be getting 10 durability off healing blocks, but as I played I found out that the effects don't stack because when I broke healing blocks, it only gave me 5 durability.

You may not sell diamonds because once you collect a certain amount in a game, they are converted into money or green cash. You earn 5 bucks for every diamond that you collect. The only items that can possibly be sold as of now are booster cards that are worthless and cannot be fused. For instance, let's say you have a level 5 (5 stars) Treasure Crates card. If you receive another Treasure Crates card by leveling up, purchasing a booster pack, or redeeming artifacts, then an option to sell cards will pop up in your cards menu underneath the Buy Cards button. When opened, you will see your useless Treasure Crates card there and you will be able to sell it. In conclusion, you can only sell the boosters cards that are useless to you for very little money.