Newbie here.
Does anyone know if the entrance to what’s left of Diffwys is still open?

I had a wander around the waste tips below Maenofferen and had a look down the Votty adit (wet).

A potential hole for Diffwys (approx 52.999345,-3.915948) led to a few featureless caverns, but not the ones pictured on here.

Depends what diffwys, there's Diffwys hen and newydd. The best one is via the big crater in top. There are adits and chamber's to go through to get inside the crater, but easiest way if unfamiliar is just to go right up road next yo maenofferen sheds and work your way straight up from there, or take the road up fromDiffwuslevel 6 mills which if you take the road to the right furthest away from maenofferen sheds you will get there that way, you just follow road up from there, once up higher than mills and carrying on up road you will eventually see incline ahead, the crater is right if incline.(and level 6 adit is right of bottom, but very shonky in there) But beware, there is only one way down into the crater and its extremely steep. My facebook page has loads of Diffwys photos in my albums, you might get bearings from those
https://www.facebook.com/profile.php?id=100039799521096 
I've got few videos i think on Youtube which may help in crater, or getting in to crater through level 6 adit.
[youtube]https://youtu.be/DA8AFw8zxUg