Nice diligence here.

A few comments.
1) I wouldn't say 6% rewards are "too good to be true" when BlockFi and Celsius offer 8% for staking. However, it's very competitive and the best for a crypto card by far.
2) The 6% requires a TERN stake which is why I'm more confident that it's related more to staking then orderbook trade. For example, I stake 145,000 TERN and that costs me $1200 which drives the price up. If i plan to keep using Blockcard its a great deal because I can profit from others staking but earn rewards short term as well.
Staking TERN at this level makes a lot of sense as it's relatively cheap.
3) The biggest issue I have with your math is that it does not accurately reflect the conversion. I can go buy TERN on Whitebit or another exchange at 0.07 and deposit it directly on to the card. As long as I spend at 0.07 then I'm always good, regardless of the orderbook.
1) Do you understand how Blockfi is able to award 8%? They loan out your crypto and charge a higher interest rate to borrowers and pass on a part of it to you. This is not the same as what is being offered by blockcard. "Unlimited" 6% cashback for just $1200 stake.
2) The TERN stake is meaningless. You can give them a $1200 fee. Then go spend $100,000 and make $6000 in cash back. How does a company pay you $6000 for staking just $1200? "Too good to be true". Also if you have to depend on others putting in money, it makes it a pyramid scheme. The day others stop putting in money, you lose your money.
3) Doesn't change anything. They are still able to conduct arbitrage in a captive market. The "spending" of TERN on the blockcard is ONLY related to USDD:TERN market which is captive where I have clearly pointed you to the ongoing arbitrage. Don't trust what I'm saying - look at the transactions yourself.
But hey, you do you!