The crown looks pretty good, (thanks for the better picture...nice camera-work!) but the youngest leaves darkening so suddenly still seems a bit odd to me (yes I know about light exposure and pigment-ing of plants...etc.), but indeed if you were cooking them under the lights, I suppose that could be it.
Some years back I too killed off a Ceph baby someone sent me, which was sad as I had successfully grown them prior to that. Actually, my plant never recovered from shipment, and I watched it do a lot of things similar to what you have experienced, not long after I got it. (Thanks to some very kind people on the forum however, I am back in the swing of things!)
Anyway, my youngest leaves also blackened not long before the crown quickly rotted out, which is why I thought along those lines.
It is true that the plant is likely experiencing a lot of stress, as the environment it has been under seems less than ideal and is constantly changing. It might have been an idea to contact the person you got it from to find out what they had going on, as it might have been easier to duplicate the same conditions that way, and then slowly make changes. It is a little late for that now.
And indeed, trying to check the roots at this point might just stress it more... or may in fact help you determine what to do next, but it all depends on your abilities! (I would, but that doesn't mean you should!) I often forget to take the capabilities and experience of others into consideration when giving advice. Sorry.
It does look better than I expected, in the close up. DO keep am eye on it, and if the small leaves grow it will be a very good sign that it will recover, especially under the conditions as others have advised. (If the growth stops and the "black" areas increase, it is a bad indication!)
One last thought on the subject of growing Cephs in LFS... Like any other plant, they can grow in a variety of conditions, and one variable affects other variables. If the soil mix is loose and coarse, it won't hold a lot of moisture and will need more frequent waterings or higher humidity. That could then also slightly affect the temps or how much light it is exposed to. When you change one variable, others change also, in order to attempt to maintain balance. Once a variable goes too far, it goes beyond the range that the plant can survive in, it then dies.
There is a "mid-range average" of what to shoot for (look at the Ceph info to see what it is), but beyond that there is some variation of what the plant can thrive under. That is why there is some variation in people's advice as to what is "best" or what they are doing to get their plants to thrive.
If someone rarely waters their plant, then a more "water retentive mixture" is needed. It is all common sense.
Also I would like to mention, it is growers like us all who experiment and find new and sometimes better ways of doing things. That is how discoveries are made, by pushing the "variables" I mentioned, to their limit, while altering other variables to find better balances that the plant likes.
Standards of what to shoot for are out there because people have experimented with mixtures and such and found out what works best for them. They become standards when others try it and get good results too.
So while it is good to follow what is tried and true, as it has been shown to work, it is also good to experiment if you are so inclined to do so and have some plants to experiment on.
We might find that glass beads are the ideal planting mix for growing Cephs on (oh, but it must be beads of a specific mixture... of specidic shapes and specific sizes!), who knows!
That is what makes experimenting and growing fun sometimes.
Well, good luck to you all.
Paul
(And I am serious about the glass bead planting mix! Make sure when you have success with it that you give me due credit!) (I myself am still trying to find a bead source that can supply me cheaply enough, that has various shapes and sizes!)