None of the players traded in similar situations signed extensions; same reason LaMarcus didn't sign an extension, the player gets so much more waiting until their a free agent.
The Lakers thought Dwight would sign long term, just as the Cavs thought Love would sign long term when they traded Wiggins for him. The Heat thought Dragic would sign with them. Love and Dragic resigned, Howard left. These things aren't binding and players can change their decisions on a whim.
There isn't any scenario the Blazers can trade for Dwight Howard and have assurance he signs here as you stipulated.