So what does this mean? They just run at you and explode or...?

IsMasochist determines whether friendly fire turns the monster hostile.
Not sure about the IsSadist thing, but it might be why the Banana Grower Encourager kicks the workers as they walk past (Banana Growers are masochists)

And I have no idea about IsExtraFragile.

That would be because you can't wish for a specific item unless it is the only one of its kind.
What you really ended up doing was wishing for a can filled with a random substance.

END is particularly bad for the danger level, yes.
Equipment you use can also lead to a much higher danger level, with certain items being worse for it than others (particularly artifact items) and increasing the danger level the more powerful they are.

IIRC actually, all stats contribute to a higher danger level, it's just that END is the number one offender in this case.
Kinda sucks the game spawned not one, but three named monsters on you at once at such an early level. I don't suppose you had some ridiculously buffed equipment?

Heheh, this reminds me of the time I modified the chars.txt so Kaethos was hostile, had enormous END but no AGI or any other stat really. Good times.

That would be:

StrengthValue = X;

Not sure about density or how it affects anything. Maybe how much of said thing you can put in a chest, or how likely to break the item is?
EDIT: Going to go with the likeliness of an item of that material breaking, considering Illithium has a really, really high value at 6000, and an easily broken material like balsa wood for example has a density of 250.

Just how likely to break the item is also seems to depend on other factors, such as how much of it is present.
For example, GLASS has a density of 2500 (only 1000 below Diamond) and a strength value of 40, but glass bottles are known to break easily - likely because a bottle is comprised of very little of said glass material, but is still at least a little impact resistant due to glass' large density rating.
Maybe it's a little bit of everything - strength value, weight, and density?

Also note that Illithium is much more dense than Diamond and Valpurium, but is entirely inflexible (Flexibility of 1) with a lower strength value to boot.
However, after running some wiz mode tests it appears than an item made of Illithium actually is slightly less durable than one of Diamond or Valpurium ("extremely strong" vs. "almost unbreakable" for the other two) which leads me to conclude that it is, in fact, a matter of both density and strength value. Density also appears to have an adverse affect on weapon accuracy - although this may depend on the weapon type, as I have so far only tested quarter staves and bastard swords.

The other problem would be finding the devs again and convincing them to work on IVAN again.
I suppose a couple of grand is a pretty nice incentive though, but then I don't know how kickstarter and the like would appreciate it essentially being a bribe

Just confirming that the healing gas I wrote up above works and is good fun. I made it a possible dwarven grenade payload.

Working on:

• Wand of Rain (causes rain of random liquids - healing liquid/blood/water/pepsi/vodka etc.)
  
• Wand of Clothing (splorts out a random piece of armor, few charges)
  
• Mushroom variant that uses mustard gas/healing gas alternately
  
• Exploding grenades / flasks. Probably possible through script editing only.
  

I'm also going to see if I can stop mystic dark frogs teleporting away your equipment if you have a ring of telecontrol equipped.

I'm inclined to agree. A second ring of fire resistance can be the difference between being killed by a landmine in GC1 or surviving with very little HP, in my experience.

Of course enchanting up a cloak of fire resistance is even better, if you aren't wearing any other sort of special cloak. Helps to free up ring slots for, say, telecontrol rings and the like.

So, the "win IVAN" challenge huh?
Alright, I've never managed it before but I suppose I'll give it a shot.