thanks for highlighting this report. It basically goes through the Mil Hdbk calculation (wrongly!) for the TWTs themselves. However, it doesn't give any insight into the EPC and/or other high power devices. I have seen a Boeing report which goes into how the reliability of TWTAs and SSPAs have improved over time but, unfortunately, gives no baselines failure rates as these are "proprietary".
I'm somewhat surprised that there doesn't appear to be any public source of information on these, what I would assume are, off the shelf components.