Finally had a confluence of ideas & an old enough pair to try adding lugs.

Grip would be provided by some conventional patches over the worn out parts & some 1/2" squares of 2mm thick common rubber where the soles were most worn.  The battle with grip has always been not making the soles too heavy.  

Another new discovery was that black EVA is harder than white EVA, on account of the pigment.  The last of the white EVA would have to be used up 1st.

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A test run up & down the mountain showed the lugs to be very effective compared to the last run on smooth soles.  Their weight wasn't a problem.

Helas, still bailed on the same 2 downhill trouble spots as last time.  The last descent on those sections was in 2018 with hokas.  The problem with those sections might not be the sandals but the lion.  If these sandals with lugs couldn't do it, no-one else would be doing it.  It's a lot faster to do that downhill section on the paved road, even though it's longer.

Would continue using 16mm soles with the lugs, to defeat the rocks.  Would add lugs to the sections with the repair patches.  That might improve confidence in the downhills.  They might replace the patches.

The previous attempt was documented thus: https://hackaday.io/project/88623/log/204326-mountain-test-1

Managed to beat it by a few minutes.

Common rubber wears down so fast, they wouldn't make daily drivers.  Wouldn't go thinner than 2mm.

Right rear had the most wear.  Applying a dab of E6000 might buy them some time.  Found bending the lugs before gluing them on can keep them from curling up.  Might make the lugs smaller to reduce the curling in the corners & make them lighter.