Maybe the difference between the eval of the best move vs the next one(s)? An "only move" situation would be more risky than when you have a choice between many good moves.
That's it exactly. Engines will often show you at least 3 lines each with their valuation, and you can check the difficulty often just from that delta from 1st to 2nd best move. With some practical chess experience you can also "feel" how natural or exoteric the best move is.
In the WCC match between Caruana and Carlsen, they were at one difficult endgame where Carlsen (the champion) moved and engines calculated it was a "blunder" because there was a theoretical checkmate in like 36(!) moves, but no commentator took it seriously as there was "no way" a human would be able to spot the chance and calculate it correctly under the clock.
Not necessarily. If that "only move" is obvious, then it's not really risky. Like if a queen trade is offered and the opponent accepts, then typically the "only move" that doesn't massively lose is to capture back. But that's extremely obvious, and doesn't represent a sharp or complex position.
