When there's no suffix, the longtime rule was, whole numbers are pF, fractional numbers are uF. So that's 1nF. :>)
That's the input coax cable.
A photodiode? No, you likely need a trans-resistance amplifier. But tell us, under what conditions do you see 100mV, into 50 ohms?
As for the amplifying capability of 74hc04 inverters: not as good as the more modern low-voltage 74LVC04 inverters, or better, for a linear-feedback circuit use 74vhcu04, etc., *unbuffered* parts. Tiny single-gate ICs are nice for this purpose, 74vhc1GU04, etc.
The diodes won't conduct if the input signal is smaller than say 3V p-p. But they do provide additional static protection.