Hi,
1) there are few RTD demo examples as well as LIN stack demo within S32DS you can refer to

2) this is rather question for Vector. I do not know and work with that tool. Maybe you can find some guidelines/videos on the web.
I assume it woulld be enough to setup used HW interface for Slave functionality with given bitrate. Then tool would be able to show frames sent by master. To respond to header requesting payload, a response frame should be prepared in the tool.
If you have a LDF file, this can be loaded into a tool and so it has info about frames to be send/received.
3) basically the code itself is running as desired, so it publishes frames successfully and receives slave responses too. If not it is good to have a scope/analyzer to measure MCU's TX/RX lines and LIN bus.
BR, Petr