What is the purpose of diodes after RF power amplifier and before LPF?
The diodes are part of a RF switch that switches between RX/TX modes of the transceiver (See also my answer to another question).
This can be accomplished by changing the DC potential at the RX LINE side of the diodes. 

You can see a RF filter of the TX LINE side consisting of R150 C174, C176 and L40. I'm sure there is a similar filter on the RX line side (connected to a switchable DC voltage source) not shown in the excerpt of the schematic. 


If the diodes are forward biased RF power can pass from TX LINE to COMMON LINE. The transceiver is transmitting.
If the diodes are reverse biased RF power from TX LINE to COMMON LINE is blocked. The transceiver is receiving.