my 2 cents worth...
regarding the 50 ft, i am not too sure but it is either because the ILS glideslope is supposed to bring u 50 ft above the threshold, or since its a precision approach, 50 ft has been added for the PEC.
and from night pistol, if its an IFR flight, then if my memory serves me well, you can only descend below the LSALT within the circling area, which for Cat A, is 1.68 nm from the runway threshold. apart from that, the usual requirements: clear of cloud, in sight of runway, min 5 km vis. ref: AIP ENR 1.5