Just unloading some thoughts on the event based on 100+ runs:
~~~~Treasures~~~~
OSash exists. There are two mobs you should look for that drop the ??? Sash: Floors 21~39 have the Gwyllgi hound; and Floors 81~99 have King Arthur. I've only ever gotten it once of the 5 times I participated this event nonstop.
??? Weapons are found in abundance in Floors 41~59, the reason being is that the seven Skull of _______ make up a good chunk of the possible NMs found there. Great for Pulse hunting if you're trying to build an Ambu weapon.
??? Boxes are typically armor pieces of some variety in the main five body parts... or random junk. While there aren't many grand prizes here, I open these anyway for lockstyle pieces. I can confirm I've gotten gear from Providence Watcher, the final boss of the Voidwatch quest line, this way.
The following does not drop: any HTBF armor in the more coveted levels (Lilith, Cait Sith, Shinryu, Odin); Omen *main bosses; any newer content, Volte gear.
Any pulse is possible to obtain, including Sagasinger.
The giant-***-table is in the ??? Box +2, same pool as the ZNM trophies/pop items, which are in floors divisible by 20.
If you feel superstitious, put a R or R/EX item in the DBox. I actually had this happen to me with a Sroda Neck back when it was still worth 20mil and I wanted to die. (I still want to die for hesitating on selling it now that it's about as worth as a pack of chewing gum in a gas stop.)
~~~~Inside Nyzul~~~~
Some square wall rooms will have it so you face a wall. Going left or right will either make it so you run straight down to the next room; or zig-zag and eat up a couple seconds of time. If you run into this room--and another room is just like it, always take the direction opposite of the zig-zag line since it never repeats itself. Example: If I go left and see that the walkway is straight down, enter the next room and be given the choice again; going right will always be another straight. This is especially useful to know when racing against time/rushing through lamp puzzles.
Over time you will notice some rooms have a very distinct pattern and you can automatically map out where you are. One such example is if you start in a room that is in a L shape with only one way leading out, you will find out you are in a small Y-shaped area, where there's a small staircase towards the left.
Animations can be a factor. A BLM II spell may be more important than, say, a Blue Mage casting Subduction if you need to kill just one enemy.
Gears detect by magic, so use a throwing weapon if you are in an otherwise almost impossible room to avoid detection (it will happen). I keep an Aureole for this very reason.
Z-Axis will mess up your AoE so be prepared for some jank. More specifically any L-shaped corridors where are erected floor panels outlining the hallway, that's where I usually have to redo a spell if I get unlucky.
It is possible for the same NM to appear multiple times, this includes the aforementioned O-Sash carriers.
*Edit: Originally I claimed all Omen gear was out of the question, but looking back I did notice a Nusku Shield was possible, so there's that.