Bit error rate calculation for LIN protocol
Everything related to the clock is configured by the software running on the MCU.
This includes what is the clock source (internal/external), the clock multiplication with PLL (if it is used or not), enabling and dividing down the clocks for buses and peripherals.
There is no way we can know without looking at the source code how your software team has set up the clocks. Ask them.
Edit to actually answer the questions:

Yes, LIN is usually implemented with simple UART, so any UART should be capable of implementing LIN. The datasheet specifically mentions LPUART is a LIN/UART peripheral.

When you know how what system frequency comes in to the LPUART peripheral, calculating the values to get 19200 baud rate is easy, and if the resulting baud rate is not exactly 19200, you can check if it is within LIN tolerance.


SPLL is the clock output from PLL. Configured in software. However, it uses the crystal as reference, so at least you know 8 MHz goes into PLL.
FIRC is the clock output from Fast Internal RC oscillator.
SIRC is the clock output from Slow Internal RC oscillator.
SOSC is the clock output from the crystal oscillator, so it's 8 MHz.
The SIRC has a tolerance of max 3.0% or 3.3%, so it can't be used for LIN.
The FIRC has a tolerance of max 1.4%, and they specifically mention it is suitable for LIN as a slave, but not for LIN master. It's 48 MHz.
So the only way to be a LIN master is to use the 8 MHz crystal, either directly or via PLL.

The UART is a bit non-conventional as it has freely selectable oversampling ratio. Assuming you use the 48 MHz clock, and if oversampling ratio is set to 16 like for other typical UARTs, the OSR=15. The SBR must then be set to rounded value of 156 to achieve closest value to 19200. The baud rate error is 0.16%. You can select other oversampling ratios to get closer to 19200.