If 3rd terms are approved, Obama would bury him in a landslide deeper than anybody has ever seen; the greatest mandate in the history of mankind.

It wouldn't work like that.

Dershowitz' rather contrived theory is based on the idea that the 22nd Amendment provides that no one can be elected to the office of President more than twice, but that the contingent procedure in the 12th Amendment (which governs if no one gets a majority of electoral votes) isn't an election. The House doesn't "elect" the President, in his reading - it "chooses" the President. So his method for Trump to serve a third term without being elected is:

1) He runs in 2028 and wins enough states to have a majority of EV's;
2) Enough Electors abstain from the Electoral College vote that there isn't a majority;
3) The House then chooses him to be President again.

The trick here is that the House "selection" is done by state, not by number of Representatives. Republicans hold the majority of Reps in 30 states, Democrats 18 (there are two that are evenly split). Even if the Democrats win the house in 2028, it is somewhat unlikely that they would flip enough of those state majorities to be able to control the House "selection."

All of this is utterly implausible. The slightly less implausible scenario is that Trump has two stalking horse candidates run in the election (say, a Don Jr./Jared Kushner ticket) and they win, and then Trump gets himself chosen as the Speaker of the House. The two stalking horse simultaneously resign, and Trump ascends to the Presidency for a third term, with no need to argue about whether what the House does in a "no electoral winner" scenario is an election or not.