Exploring the serial I/O of a 40+ year old uP / MPU / SBC / TTL device

What properties and values do I have to use in E.setClock(...) for using Serial2/USART2?

USART2 is on PCLK1 according to the reference manual, so you'd need to use PCLK1:4 to halve the clock rate of that.

I can't be sure where USB gets its clock from, so there is some possibility that lowering the PCLK1 speed will break USB - I'm not sure