Maybe I can help clear this up....maybe I cannot.
There are 2 issues here. 1. Approach naming and 2. Approach clearances.
1. Approach naming - There was a change to this a few years back and the current ICAO convention is to name the approach according to the navigation aid or system that is providing AZIMUTH guidance. This is why all the DME's were removed form the approach names. Of course the issue that will be arising in the near future is what do you name an approach that starts as an RNAV/RNP and ends in an ILS.....ICAO are still sorting that one out.
Part b to this was that someone declared (and I think it was CASA) that if there are 2 DME's available for a particular approach, then we would have to provide a separate approach for each DME. This is where the ILS-Z/Y's are coming from. My thoughts are that the intent of the Y/Z suffix was to depict approaches that gave separate tracking...like a timed reversal vs a DME reversal or 2 distinct inbound tracks. The result is that we are publishing 2 ILS approaches that are exactly the same on 2 different plates. This is further complicated by PRM...so you have ILS-Z/Y and PRM-Z/Y
2. Approach clearances. Up until a few months back I was told that the approach clearance had to contain the approach title when it was issued. With recent questions to ATC we have become aware that, in the case of Sydney at least, for ILS approaches they just say "Cleared ILS approach". They do this for a few reasons but the main one is that they just don't give a rats which one you fly. If your ILS or PRM clearance does not contain the suffix Z or Y then it is up to you. If you look at the AIP GEN 3.4-45. There is a note in the left column. "...the procedure suffix may be omitted when no possibility of confusion exists." so no-one is breaking any rules here....its just more complicated than it needs to be.
This scenario is being further complicated with the introduction of CAT II/III minima and the insistence that these also be published on separate plates (ie YMML)
The future...CASA has been approached with a solution, which a safety case is currently being compiled for. The proposal is to publish 1 ILS plate, and provide refences to the 2 DME's on it. Provided we can keep it simple I think it would work and we are aware that this is done in overseas locations.
Hope I have answered some questions.
Alpha