Calvin is in a bit of a slump this year. Could just be randomness, or it could be the new offense isn't that great. I would consider trading him for the right price.
I would take that second trade in a heartbeat. Ball just isn't that good, the upgrade from Ball to Murray is enormous. And if Murray gets hurt, his handcuff backup will probably have good production too since Dallas' OL is so damn good right now. In fact, I bet that even if Murray gets hurt, his backup Joseph Randle would score more than a healthy Ball would.
On the other side of the coin, dealing Megatron for Demarius is not much of a concession. Remember, these were the 1A / 1B WRs at the start of the year. Megatron might be worth 3 cents on the dollar more than Thomas. Both have been disappointing this year. I think you could trade them straight up and it wouldn't even be a terrible trade, especially since Thomas was obviously impacted by the Seahawks game and isn't being held back by a new offense like Calvin is.
I think that's a very generous offer by your trade buddy. He must be terrified of Murray getting hurt and is trying to sell high. Let him. As long as you have Murray's handcuff, this is at worst a break-even trade for you, and best case scenario it's a huge win.