TMS570LS0332: Crystal Kelvin Connection

Ok. The first point is clear now. 

Second point still unclear. I do not find a layout recommendation for the crystal.

If we have a PCB with 4 Layers for example, and the crystal is on top, should the 2ND Layer be ground or a Kelvin GND island?

You don't have to add a Kelvin island to the 2nd layer, or the 2nd layer doesn't have to be GND layer. Please place the crystal close to MCU, and keep the clock traces as short as you can. 

But can we have a GND layer on the 2nd layer or is it mandatory not to have it? What about layers 3 and 4 Could we have GND layers? 

If you can route all the signals on the top layer and bottom layer, it's good to have GND plane and power plane in between. The standard 4 layer PCB stackup is:

Top: Signals

Inner 1: GND

Inner 2: Power

Bottom: Signals

The GND and Power can be switched depending on the layer with more signals.