Tether was founded by the same person who created Omni -which, by the way, is an awesome and underrated platform-.

Originally, and on their website, Tether states that they have been offering USDT -a USD pegged token- and EURt -an EUR pegged one-. USDT has been working since many time ago. EURt, although also tradeable, isn't.

The problem is the price which people must pay in OmniDEX -the decentralized exchange offered by the Omni Foundation-. In order to buy 1 EURt, people must pay 0.2 OMNI, which translates into a 3.09 EUR price for each EURt. At least, that's the market price.

In summary, they are issuing cryptocurrency tokens, each one supposedly backed by one USD.
This looks and smells like a security! Why the SEC haven’t started investigating, I am not sure.

You are for sure more aknowleadgeable than me, but if the only possible exchange for every single USDT is 1 USD, then I don't see any possibility of make any profit: the person will get 1 USD, doesn't matter when the exchange takes place.

And finally, the most recent report on the Tether website is from September 2017 - why nothing since then?

Because that was their last report as the auditing form for Tether.