Vintage Denon Direct Drive Turntable

(1) I have done the thought experiment as regards threading the slate. I don't think it would work. I am not going to try. I think the only choice with slate is to just drill a hole thru and then use a nut and bolt. There IS some appeal to the idea of threading the slate, because it would make for better coupling in theory. But I think the slate would just crumble if you tried to tighten down the screw. You might experiment with a scrap piece of slate, though.
(2) If you have the slate slabs honed, the sides would be plane parallel to each other and there would be a close fit between the two pieces. Most mills can hone the slabs, at least here in the US. (I had it done in Vermont and in Pennsylvania, by two different companies.) I don't think you need adhesive, but I do think that sorbothane is a bad idea. It would essentially de-couple the two pieces of slate, when what you want is good coupling. Just my 2 cents.

I've had a look at a similar thread on another forum, and agree with you Lewm that bolts are best for this job! I have decided to bolt both chassis and tonearm through the top layer only, I will drill a rebate/countersunk hole into the underside in order to secure a washer and nut, that will remain flush when placed on top of the base layer.

It turned out that in my stack of un-plinthed TTs, I had a DP-80 that I had picked up a while ago. This morning, on a whim, I put it in a solid wood DIY plinth that someone had built a number of years ago with a couple of SAEC armboards, and sold on the net for $50, and I installed my SAEC 407/23. Using the old "eyeball-it-and-guess" set-up technique, I got a wonderful TT in about 10mins. It has great stability, and I was reminded yet again how good the SAEC arm is. Upon returning home this afternoon, I spent a bit more time on set-up, and it turns out I was really lucky the first time. After several records, I continue to be surprised at how good it is. I think I need to look into a better plinth, but I am going to keep this TT in rotation for a little while.

T_bone, Raul used a minimalist plinth like that with both an SP10 and a DP80 and liked the results. It would be interesting to use your first effort as a reference, if and when you try to build a heavier/denser plinth for the DP80. I think the DP80 and other top line Denons are under-appreciated in the recent upsurge of interest in DD tables of yore.

Just a quick update.
I've handed over my template to a fella who runs a mill and specialises in slate. I'm going for a monster one-piece 100mm thick 'headstone finish' type slab. The cut-out is tricky (especially at the front where it comes with a couple of inches of the edge, and it will have to be cut with a water-jet, but I'm hoping he'll come through. At the moment I've decided to go for threaded inserts glued into bore holes for the anchoring points.