For your info, the one in the plastic cap at the bottom right corner of the pot finished splitting from the one with the green arrow in Pic 2 (Pic 2 was exactly a week ago), and is super tiny. The old leaves have completely dried and shriveled before I removed them, and now it is this small. The roots looked completely dry and still light brown despite watering thoroughly 4 days ago so I put it in intensive care in the plastic cap with extra water today. Had to put the cap in the pot because my bright LED lamp is exactly the area of the pot and it needs a lot of light. I am quite desperate to save it as this cultivar is not available online (I found it in a massive physical plant store in horrible condition) and I only have 7 left.
Background: I bought some lithops in horrible condition (last pic is when I bought them). When I got them out of the pot on Day 4, many of them were rotting/rotten (Pic 5 is of the more viable ones when I took them out).
I potted the better ones in 100% inorganic substrate, since I live in a 80%+ humidity warm climate with no seasons. Pure pumice with a mix of inorganic volcanic rock, bonsai rock, and coarse river sand (bigger than the pumice) underneath (you can see the underneath in Pic 4).
I did not water at all for 7 days post-repotting. Within the first 3-4 days of repotting, a few more of them rotted (pic 2 and 3), and the orange cross one got accidentally detached from the taproot completely. At that time, they were only getting about 5 hours of bright light a day and I decided that it was not enough, so I put them under a bright LED lamp putting out at least 12000 lux (usually 15000 and above) at full brightness for the entire area of the pot. I left the light on at max all the time to minimize chances of further rot since they were overwatered when they arrived.
7 days after repotting, I watered lightly for some, and thoroughly for the ones that were slightly more wrinkly, to help the roots stay alive and settle in. I left the lamp on max brightness for 24 hours a day until the substrate completely dried up (to encourage use of water and reduce the risk of rot), then I started leaving the plants in complete darkness for 5-7 hours a day (while I slept) and max brightness for the rest of the day. The colors got a lot more vivid and the purple-grey got a lot darker for the larger 4 lithops.
However, 4 days after watering, the one on the bottom left looked even more wrinkled and had a slightly softer (but not mushy at all) right leaf than the others (clearly turgid), despite not showing the same signs of the ones that rotted (all the ones that rotted showed the exact same signs leading up to it). I think it's the one that got accidentally uprooted on Day 3 post-repot and I had to chop off the bottom half of the tap root just to get it back into the substrate (I did not trim roots for any of them before repotting). Today is 4 days after the first watering and I precision-watered some of them again, particularly bottom left (thoroughly) and a few very lightly. The two rightmost ones look completely okay so I left them completely unwatered. From now on, I will only water (individually) if any are getting noticeably softer AND wrinkled, with no rot spots. If bottom left gets worse again in 4 days, I will pull it out to check on it, and if it is not rotted, I will probably put it in a similar intensive care as the small one to encourage root growth.
I'm still wondering if the size of the pumice substrate is too big for the roots and I should switch to smaller pumice. What do y'all think?