The requirement for fast acquire and the one to "indefinitely" ride
out ref clk loss seems "dissonant".
I could suggest that you want some modality there. Like (say) a
loop filter that is very fast, unlocked but very slow, once locked;
also a "activity detect" on the ref clk (and maybe feedback too)
which will disable the charge pump (or its digital accumulator
equivalent) so that "last correct" state is held. Probably ought to
throw a flag though, just so somebody above, knows.
If it's all-digital then holding the freq should be pretty easy,
but getting to the actual holding can only happen once the
"no tone" decision is made, which would be maybe 10 cycles
for a variable frequency range, fewer if it's "8kHz or nothing,
in a heartbeat". But making things not flake out from lesser
spurii wants some idea of what those might be, short of total
and permanent / long term clock loss.