Some points of confusion I suspect ..
(a) the aim is to maintain the risk levels of the original Type Certification.
(b) this means ALL the design standards, many of which are not known to the typical flyer, have to be included in (M)MEL assessments
(c) if widget A (not being in the (M)MEL) is broken, then the (M)MEL (or defacto) stakeholders have to assess what restrictions, if any, are required to permit ongoing operations at a similar to original risk level. The MMEL provides guidance to the operator producing the MEL, the MEL provides prescriptive minimum guidance standards to the operating flightcrew and maintainers. In some jurisdictions (Australia for instance, with CASA's GMEL system) the regulator gets involved at the MEL design level to simplify the whole exercise for everyone.
(d) risk assessment involves probability and consequence of occurrence, and period of exposure to the risk .. hence the time limits on MEL items
(e) items which do not affect the airworthiness of the aircraft...
the problem here is that the typical flyer is not well-placed to assess this question, other than for day to day operational considerations .. refer (b)
(f) the MMEL is not exhaustive in that only those items considered "useful" are likely to be included. There is no reason why an operator cannot present an argument to the relevant regulator for additional or varied permissions when considering the (M)MEL