OK, Lessee. It took me a while to figure out what measurement basis he was using (English or metric) and then do some calcs:
12mm = .472"
14mm = .551"
So, from the photos, and I'm reading the bottom scale of the caliper which I'm assuming is in centimeters, from the "zero", not the end of the sliding rail - top photo to bottom:
Top Photo, head flare depth: .8cm = 8mm = .315" (about 5/16" ) Don't know the angle.
2'nd Photo: thread length: 2.6cm = 26mm = 1.02"
A 4-bolt hub, VW wheel bolt has .75" or about 20mm of thread for the steel wheel thickness but it's a hex head on top of the flare cone - Slight apples and oranges, there so I went with the threaded shaft length measurement for both.
3'rd photo, head flare diameter: 2.7cm = 27mm = 1.06"
4'th photo "Image not found"
5'th photo, thread diameter = 1.4cm = 14mm = .551"
It looks like a 1.5 pitch thread, the same as a VW - that is not a "fine" thread. I just measured a VW wheel bolt and it's a 1.5 pitch, and my VW wheel bolts are 14mm (.551").
I would buy 'em from the UK or Italian places. I just did a quick look on my usual three sites (McMaster, Fastenal and Tri-State) and came up with Bupka in that size/shape/pitch. I remember looking for bolts similar to those in the US a while back and gave up, taking a different route (but my spacers are 1-1/4" thick, giving me those options).
12.9 bolts for that application is fine. Get the 25mm length, if you can, but as you say, there's nothing back there to interfere with them if they're a tad longer (and you can always shorten them with a cut-off wheel).
BTW, the overall bolt length (above) is 2.6cm (bolt) + .8cm (flare) = 3.4cm = 34mm so maybe they're specifying a 35mm bolt overall length given the thickness of their adapter metal. Be careful on that.
So, my advice to get it right the first time, is to take an existing wheel bolt that you're replacing to a mechanic or a machine shop and ask them what it is for size, pitch and length and order that.