I like the ideas behind this. Insanity is currently a rather boring death sentence. The misuse of psychiatric terminology rankles me, however, so I shall be forced into a crusade of nitpicking for a few of these.
Multiple Personality Disorder- dwarf becomes two dwarves. Skills divided between them. Name changes at random. Extra-fun if one suffers from cave adaptation, and other from agorophobia.
Multiple personality disorders (actually known as Dissociative Identity Disorder) actually never present themselves with only two personalities. It's normally somewhere around five or more, and it's also a mostly Western disorder (suggesting that culture might play a large part in it.)
Having several personalities but also keeping skills the same between personalities would make more sense from a realism perspective.
OCD- You can do anything with this one. Easiest would be "Dwarf faints at sight of dirt, vomit, vermin or miasma". May also refuse to eat anything but certain foods, or ignore all but a particular type of stone when it comes to working. To make it interesting, have this one couple with rapid skill advancement- so your legendary stonecrafter may also be the hardest person to keep working.
It's often characterized by repetitive activities like washing hands. Spending whole DF days going through bar after bar of soap would work well here; artistic dwarves could throw away their half-finished creations randomly too (but normally have a higher-than-average quality.) I'd suggest random selection of a few traits here since it can vary.
Schizophrenia. Non-noble dwarf believes he is noble, comes up with a list of requirements and demands, and refuses to work. (Or hey, maybe that's just where nobles come from)
Schizophrenia does not equal multiple personalities (I'm not sure you made that claim, but the misconception's prevalent enough I'll make it here.) It comes in a few varieties, some of which I'll describe: paranoid schizophrenia, where the victim is highly suspicious of basically everything; catatonic schizophrenia, where the victim frequently stands motionless for hours; and disorganized schizophrenia, where the victim is more generally bizarre-thinking and also doesn't show much emotion.
Hallucinations, delusions, grossly disorganized behavior/thought, blunted emotions, lack of speech, and lack of motivation are also common between all types (and two of those criteria are needed for diagnosis.)
Amnesia- dwarf loses all skills.
Short-term memory loss- dwarf is unable to advance any skills.
People with amnesia can still learn and use other types of memory. For example, Clive Wearing, a guy with no ability to record short-term memory into long-term memory (anterograde amnesia) could still learn to play the piano, and would do so despite being consciously sure that he couldn't. Conditioning also doesn't use the same neural pathways that more normal memory does.
More likely than not, automatic skills that rely on much conscious thought (fighting skills or many other physical ones) wouldn't be greatly affected, whereas other kinds (social or creative skills) would be.
And because I've got Inception on the mind, one of the director's earlier films (Memento) has a very realistic portrayal of the protagonist's anterograde amnesia and designs the whole movie around it.
ADHD- dwarf randomly reassigns his own work orders.
Eh, I'd be more prone to making them very susceptible to distractions. Like having craftsmen be distracted by other sources of noise, causing them to work much slower, and/or having recent thoughts screwing with work quality too.
A part of me thinks these dwarfs would make terrific managers. They'd have your fortress organized in groups of three, six, and nine. It would be nice.
.... Until they went nuts and stabbed somebody for messing up the system....
I am suddenly reminded of that episode of Sayonara Zetsubo Sensei where the "exact" girl was slicing a cake into perfectly even slices for the 5 people who were going to eat it, when another person showed up, so she sliced the fifths into sixths (technically, 30ths of the original cake) using a compass and some scratch paper, then another person came, so she wound up slicing it down to 210ths, and before people could complain, someone else came, so she freaked out, dumped the whole cake into a blender with some milk, and served everyone a cake smoothie.
Okay, the coincidence is disturbing. I just saw the first few episodes of that last night.