Would a demonstrated failure rate of 0,001% per 10 000 operations (one failure per billion operations) at the 90% confidence level be a reasonable failure rate to expect from a Hi-Rel relay ?

NO. A 0,01% per 10,000 operations failure rate at the 90% confidence level is considered to be about the lowest failure rate possible with the present state of the art. As a point of information, 23 100 relays, rated for 100 000 operations each, would have to be tested with no failures in order to demonstrate the 0,001% failure rate. It is very unlikely that a relay manufacturer would test and destroy many relays just to demonstrate a failure rate. Most likely, the accepted method was not used to calculate the relay failure rate.