Related to this, I am confused why the "20V isolated NMOS FET" is listed as a separate device. It seems that any nmos device can be built as an isolated version and it should not require a different model

It's a separate device because of the 20V, not because it's isolated. All of their 16V and 20V devices have very specific layouts. All the "regular" FETs like the 1.8V and 3.3/5V FETs use the same model whether isolated or not. The 20V device requires the deep nwell, so it's part of the fixed layout.