Don't mind at all. I'm a bit rusty but I'll explain the best I can. It is not because they are diesel. A lot of twin engine aircraft have engines that rotate the same direction. It tends to be cheaper. It simplifies design costs, manufacturing costs, maintenance/part costs, etc. For example: Instead of needing a left or right dipstick to check the oil its the same dipstick for both sides.

You are correct that the critical engine is a function of both engines rotating the same direction. Has nothing to due with them being diesel. The acronym I remember from my flight training is PAST - P Factor, Accelerated Slipstream, Spiraling Slipstream, and Torque.

P-Factor: The descending prop blade produces more thrust than the ascending blade when you're at high AOA. The left engine has the shorter arm from airplane CG. So the right engine still operating will cause the greater yaw and require more control input.

Accelerated Slipstream: The descending blade of the prop causes the air on the wing to be further out and back which causes more drag on the right wing. While the descending blade on the left causes air to hit closer to the aircraft CG.

Spiraling Slipstream: The airflow produced that hits the rudder by the left engine failing does not help with control while the right engine failing helps.

Torque: The props rotating produce torque. In a single engine operation you "lift the dead engine" to help counteract the roll induced. If the left engine dies the torque produced by the right engine causes the left wing to drop so you require more input to lift that engine. If the right engine dies you require less input.

Hopefully that makes some sense. It's been a lonnng time since I've studied OEI operation. If you look up airplane critical engine there are a ton of good diagrams out that there help explain it and simplify it.