Dick,
I found a CASA document which explains some of the questions you are asking. It is titled ‘CASA’s methodology for validation of Baro-VNAV instrument approaches’ August 2016.
My reading of your first post implies that you want a seperate RNAV approach called the LNAV + V. There is either LNAV/VNAV (Baro-VNAV) or LNAV. LNAV + V is an additional feature of your avionics which provides vertical flight path advisory in the form of glide slope indications to the LNAV approach. Therefore; if you’re lucky enough to have this feature fitted, then by all means use it. As you noted, nobody is saying you can’t do this.
But at the end of the day, you are flying an LNAV approach and need to abide by all the requirements (step heights).
The problem with adding LNAV/VNAV approaches everywhere is that the requirements are more onerous, and thus more expensive. Specifically, 3.4.3 Obstacle monitoring and aerodrome operators (of the above article) notes the additional ground surveys or aerodrome management training required.
In addition I believe (reference not handy) LNAV/VNAV requires a local source of QNH - therefore an AWIS/ATIS etc fitted at the aerodrome.
Also noted in the above document is paragraph 6.2 Flyability and navigation database coding. Paragraph ‘LNAV coding of VNAV path angle’ indicates how coders (Garmin) design the advisory vertical guidance.
If you wouldn’t fly your LNAV/VNAV off your glideslope, then how do you fly your ILS approaches? Somewhat a rhetorical question as I think the hang up is on the use of the word ‘solely’.