> In the list of Pict kings (Chronicum Pictorum) there is a "Bredei
> filius Bili", also shown as "Brude filius File" who died in 693. Does
> this not demonstrate that the "b" has mutated to the fricative "bh"?
No. There is 1. no reason why /b/ should be lenited to /bh/ after a masculine nominative singular, and 2. /bh/ is not spelt
<f> in early Irish.
> He is followed shortly thereafter by "Bredei filius Derelei" and
> "Nechtan filius Derelei", which I would interpret as "son of daughter
> of Beli", where the fricative "bh" has now mutated to be silent.
/b/ does not mutate to zero.