HI SD
A few things to throw into the mix that may help, or possibly may confuse you some more!!
On point 1 you say often a CDI / HSI is interfaced to the GPS. In fact to be able to certify the GPS functionality for IFR use (and hence carry out things like LPV approaches) a CDI / HSI is a requirement.
On point 5, WAAS capable equipment is required for LPV approaches but NPA approaches can be carried out on non-WAAS GPSs. Many aircraft have been approved to fly NPAs by showing the installation is compliant with the information contained within CAA publication CAP773, although in recent weeks this method of approving NPAs has been recinded on new applications.
On point 8, things get interesting. The installation of a GNS430W (single unit only, dual installs are always STCs) can be carried out via EASA Minor change. Previously (until LPV) this could also include the IFR approvals for the GPS for BRNAV (Enroute) and NPA operations either as part of the Minor change or as a retrospective Minor change (to remove an existing GPSs 'VFR only' restrictions). Part of this IFR approval includes checking that the GPS has a CDI installed with mode selection annunciation, that it is connected to an encoded altitude source as well as checking software levels on the unit.
However, currently to approve a GPS installation for LPV operation is via an EASA Major change (STC). For something like a GNS430W this could be considered the ludicrously expensive part of the work
To muddy the waters further, the new touchscreen GTN750 / 650 units (GNS series replacements) were decreed by EASA to require an EASA Major change for every installation. Garmin realised that this effectively killed the possibility of sales in Europe of the new units and set about validating the FAA AML STC with EASA, thus allowing a pre approved EASA Major change to be used to install this equipment. This has now been granted by EASA for hundreds of aircraft types and this STC includes IFR approvals including LPV, with the caveat that an EASA Minor change should also be carried out for each airframe type to cover off the non-generic parts of the modification.
This effectively means that LPV approving a GTN series unit is via a free Garmin provided STC with purely a relatively cheap EASA Minor required to be paid for to install it, whilst the GNS series unit would require a Minor to install it but a chargeable STC to LPV approve it - simplez huh!!
I would suggest a call to your friendly DO / Installer to get prices etc as you can see its not that straightforward, the guys at Fairoaks have a lot of experience in getting through the certification maze.
Hope this has made it all crystal clear !!!!