Erik Karlsson is a free agent, so youd be trading his negotiating rights until July 1 (when free agency begins). I don't get why Tampa wouldn't just wait until July 1 and get him without giving up Miller.

No, I did a sign and trade w/ EK to give him the 8th year and a lower AAV for Tampa. Sharks sign EK pre July 1 and trade him to Tampa post July 1 (and not have to give the Senators another 1st (which was a condition to EK's trade to the Sharks - to no be on an EC team during the 2018-19 season).

Sign and trade Erik Karlsson will be more beneficial for the Sens than the Sharks lol

No it wouldn't - see above

Sharks re-sign EK on June 30th - last day to get that 8th year.
Sharks trade EK to Tampa on July 1st (start of the 2019-20 season) - won't have to give up another 1st to you guys :-)

That extra 1st only comes into play if the Sharks trade EK to an EC team during the *2018-2019* season, which ends June 30, 2019...

Sharks would lose a 2nd round pick for signing Karlsson.

Tampa would have absolutely no interest in doing that. Losing JT Miller (who's a guy that can gather some good return for Tampa) for a guy that is now pretty obvious that will hit FA on July 1st?

And I could see Tampa thinking about getting something like this done (to a way lesser price, obviously) if Erik Karlsson was 25/6 years old. But the only concern about Karlsson's contract will be his term, so Tampa has no interest in getting the guy signed to a 8 years contract.

Oh, and the sharks also are not willing to do that either, as the'd have to pay Ottawa an extra pick.