I'm surprised Gwynant as you have been on so many walk in the area. It is on the old Roman Road of Sarn Helem (Helen's Causeway) but that starts at Pentre Du on it's way over to Dolwyddelan.
I've even given the name of the village in a cryptic clue, albeit in English, Hill and meadows

It's the only photo I've ever seen of the village before it was abandoned in the 1940's. I'll put you both out of your misery as you are so near and tried so hard.
The shop was in the now abandoned village of Rhiwddolion, not far from Llyn Elsi. The buildings were pretty old and an inscription on a fireplace is from the 1600's. Here are some photos of the village now and if you had of seen them earlier you'd have guessed it straight away.