I'm not that au fait with the whereabouts of places in Llanrug so hopefully someone else will come in on that score.
The 1891 Census is Llain y Gro.
Looking at the 1901 Census and some of the other places I think it may be on the road fronm the Square towards the Church.
Put Llanrug in
www.multimap.comand the map should come up with the area known local as Y Sgwar (the square) circled.
Zoom in and you will see a road called Ffordd Glanmoelyn leading off it. You will also see a road off that called Bryn Moelyn, now a housing estate. These two names appear on the 1901 on the page after Pant tirion.
I've no idea where Llain y Gro is.
Re the Church. If you follow the Ffordd Glanmoelyn to it's junction with Ffordd Minffordd and carry straight on down the unclassified to the fork in the road, take the right fork and the Church is there on the left.
I don't know the opening hours etc re the Institue. If you put Institute+llanrug in google one of the first hits should be a site showing a photo of the building by Eric Jones who has taken numerous photos in Arfon. There's also a map there which I see shows the Church as well. If you click on the 'find more nearby' next to Erics name under Photographer you will numerous shots of the area. I would be surprised if the Old Smithy on page two wasn't the Hebron Smithy shown near Pant tirion in the 1901 census. Page 5 has a few in the Churchyard.
If you Pm me your email I will send you the photo of the grave and of the memorial in the next few days.
Re Dinorwig Quarries. Gwynedd Archives have a large deposit of their records. I cannot think of any where you can easily look up a name to see if he worked there. Thousands were employed in the quarries and this is reflected in one of the Accident Books which has over a thousand entries per annum. You may find him in there but you would have to look hard. It's worth ringing the Archives though.
Gwil