My understanding is that many lobbyists (if not nearly all of them) meet the definition of a "foreign agent" but have not registered as such.

The "how" Russia interfered with the 2016 election is an unproven allegation, and more importantly an unchallenged allegation. The Russian interference was also only looked into with regards to how it benefited Trump, and potential Russian interference that potentially helped Clinton was not looked into, even though the Steele "report" is prima-face evidence of foreign interference that benefited Clinton.

According to the FISA warrant to spy on the Trump campaign via Carter Page, many foreign governments have attempted to interfere with our elections for generations, so to say that "Russia interfered with our election" without additional context is dishonest. The intent of the Russian interference was not even to benefit Trump, it was to sow discord within the US.