In this mock the Lions are doing plenty of wheeling and dealing in the first round, so I will run through both the trades that get the Lions to the 19th overall pick before going into detail on the selection.
First trade with San Diego Chargers
The first trade of the day is a first rounder swap with the Chargers. San Diego will at least want to leapfrog New England and Miami to get the best mid first round 3-4 prospect (Cameron Jordan, J.J. Watt, Aldon Smith or Robert Quinn), and maybe even jump ahead of the Rams to steal Julio Jones from under their noses to add more depth to their receiving corps.
For the Lions, they do not really need anyone who carries a 13th overall grade. Instead, they will want to trade back towards the 20th pick. Therefore, a trade with the Chargers fulfills this need.
In my estimation, the Chargers will give up three picks to move up to 13th overall. These will be the 18th pick of the first round, the 29th pick of the second round and the 18th pick of the third round.
Second trade with the New York Giants
Now that the Lions are selecting at 18th overall, it is likely that of the offensive tackles only Tyron Smith will be off the board. Now, while the Lions will not really care who they got out of Castonzo or Gabe Carimi, the team picking directly after them, the Giants, will. They will have their eyes firmly set on Carimi as a powerful mauler in the running game, which is a big part of their offense.
With this in mind, the Lions could trade picks with the Giants in exchange for the Giants' third round pick.
This trade will work for the Lions because they would be happy with either tackle. Castonzo is a better pass blocker but can still hold his own in the run game, and Carimi is the opposite.