AlphaCentauri
As to the questions regarding the use of Area QNH. As part of the procedure design process we are required to check if the aerodrome has a TAF service. If no TAF service available then we incorporate the extra 50ft into the published minima.
By "we" I assume you are talking ASA and that is what the DAP intro para 1.11 refers to.
Any chance of the "we" letting Jeppesen know so they can add that little note into their "interpretation" of the AIP/DAP.