BlockTrades is an exchange, and most of the accounts you have listed here are likely ones that belong to their exchange. It is actually a smart business practice for an exchange to split their funds across multiple accounts, to reduce the risk of potential hacks. I don't see anything about BlockTrades having multiple accounts that in of itself speaks of anything evil..

Is it possible that all of these linked transactions can be explained by users who are buying and selling Steem via the BlockTrades.us exchange? The ones using the same memo are obviously the same user, but not everybody sending funds to and from those accounts is using the same memo. Lots of different users are sending and receiving funds to the exchange accounts. For example, even @ned sent funds to @alpha a while back..

If you go to BlockTrades.us and create an account, you can buy a token like STEEM/HIVE through their exchange (STEEM is currently disabled for obvious reasons). If as a user I want to buy STEEM with another currency (such as BTC or ETH) and have my STEEM sent directly to Binance, I am able to put my Binance memo into the Blocktrades exchange site, and buy the STEEM from Blocktrades.

(The image currently shows ETH->HIVE, but ETH->STEEM would work the same way when enabled.)

If I were to submit this trade, Blocktrades would fill the order from one of their accounts, and send my STEEM to the Binance exchange with the memo I supplied. You would see a transaction coming out of one of their exchange accounts with my STEEM going into the exchange using my memo.