Dimona.
You don't say whther the engine is a 912 or a 912S as generally we talk about the type as a 912 but of course there is a difference. The compression ratio for a 912 is 8 something to 1 and for a 912S it is up around 10:1. I would not expect the extra energy value of 50/50 to damage a 912 as musch as a 912S which is basically the same engine.
I would certainly flush the oil two or three times turning the engine over without plugs. While diesel does contain lubricants it also mixes with oil and lowers the viscosity. Your first hurdle is oil pressure and you certainly can't use the engine for anything until you have that fixed.
You also did not say whether the engine actually overheated. Since 912 engines are dual cooled I am guessing you have a water temp gauge. How high did it go? Do you have an oil temp gauge? If so how high did that go? (I have seen 220F on my water gauge.)
If the engine was seriously overheated then you may have cylinder damage but it would not usually come just from carbon build up. Those of us who worked on cars before the modern era of clean fuels etc used to spend quite a bit of our time 'decoking' our motors. The carbon builds up mostly in the middle of the piston but around the edges it does not affets the cylinder wall at all. It also builds up on the backs of the valves (as well as on the front) and eventually in the seal area (the seat.) we used to scrape that off and then reseat the valves by twisting them with a fine grinding powder in the seal until the faces were smooth and matching! However that was after 100K miles, I wouldn't expect anything dramatic after 15 mins! Having said that modern machines are a mite more pernickerty than those of the sixties. I have just seen a set of plugs in a VW utterly destroyed (electrodes burned to globs of welding splatter) in 300kms because the dealer supplied the wrong ones.
You also don't say how many hours your engine has so any comment on the causes of lost compression is just speculation. The oil consumption appears to show something amiss. My 912S is just a baby (150 hrs) but uses absolutely no oil at all between changes. Nada, none in 50 hrs.
You might certainly look for oil leaks around the important seals, crankshaft etc. If viscosity was lost I'd expect the seals to be the first thing to suffer.
I know of two or three 912 series engines that have been in "the lake," hauled out, dowsed in oil and then dried out and seem to work fine afterwards. Not perhaps my choice. Another thing to check after an overheat is the ceramic water pump seal. Mine dripped steadily for a while but it seems to cure itself if not too bad to start with.
By the way fuel injectors etc not affected by diesel in a 912. It's got carbs so less likely to have too much energy in the cylinders.
912 engines run on both Auto gas and Avgas, How long you had it actually running and under load with the 50/50 might have a bearing. You don't say, big circuit, long warm up etc?