> Can I start reading data a byte at a time until I determine the incoming message length, and then use this read routine to read the rest of the message?
Yes, you can. The bytes received are buffered in a HW FIFO, so invoking this function will not* reinitialize the receive. Calling this with a NULL pointer for buffer will dump (parameter) length bytes from the HW FIFO.
> And does this call return control to the OS until all the data bytes requested are read and placed in the specified buffer?
This is a synchronous call and will return from the function only after (parameter) length bytes have been read if length is <= 16. Behavior is undefined if a larger length read is attempted.
> Same basic question for the write routine. If I just pass the location of the start of the message buffer and length, will this routine transmit the message on the peripheral uart?
Yes, and this is a synchronous call too. Behavior if more than 16 bytes (or less if there are un-transmitted bytes in the FIFO already) are written is undefined. Use P_UART_TX_FIFO_IS_EMPTY() to check if the TX FIFO still has bytes.
* Edit
