I've had another scout around Llandanwg and Llanenddwyn again this evening.
Like you I have found quite a lot of Llandanwg images which are indeed from Llandanwg. So I agree, Llandanwg records are definitely there. I went page by page thru some of the sets of images til I got to the beginning or end of the film where I came to the record of the Mormon church filming of the registers.
e.g. one for Llandanwg (and it really is of Llandanwg) reads:
Microfilmed by The Genealogical Society, Salt Lake City, Utah at :
Meirionnydd Record Office Dolgellau
Operator: Malcolm Twigg
Date Filmed: 15 Nov 2007
Llandanwg Images
Locality of Record : Llandanwg Parish Church
Title of Record: Baptisms, Marriages and Burials
Item : ZPE/8/5 1725 - 1812
These Llandanwg records 1725-1812 seem to be OK, all consecutive years and just as you would see them on film in NLW or Dolgellau. Also you can search for them in FindMyPast and the images for Llandanwg seem to show up as Llandanwg.
I've found the same info at the end of films of some of the Llanenddwyn and Llanaber records and the bapts for Llandanwg 1902 onwards as well and these also seem to be ok ...... but there will be other films of all there parishes and I haven't checked them all - very tedious to do.
However, I'm pretty sure that although the FindMyPast list for Llandanwg says they have records for Llandanwg up to 1912, I don't think they do. I can't get any images of Llandanwg 1812 - 1902. I think they have a gap between 1812 and 1902 where the records start again. Also a big batch of the marriages for Llanenddwyn have been misclassified and come up as Llandanwg.
Were the ones you checked for Griffith Pryse and Anne Owen marriages that should have been Llanenddwyn but showed up as Llandanwg? If so, it seems to fit what I think might be happening: where FindMyPast have correct images for Llandanwg they come up as Llandanwg in a search (as they should), but you don't get all the Llandanwg ones (cos of the gaps between 1812-1902) and you get some Llanenddwyn marriages pretending to be Llandanwg.
I think all the incorrect ones I have found are Llandanwg marriages. Have you found any incorrect images for Llandanwg which are bapts or burials? It wouldn't be too bad if we could believe that baptisms and burials for Llandanwg were accurate.