1) no, but it counts as blocking sight pathing for monsters if the path is completely blocked off, thus setting the monster into idle out of chase mode. This may sound confusing, but the monsters cannot see that you are there, yet you qualify to be in spell range regardless (example: Seyren Windsor. He won't chase you because you are out of sight range, but he will spear boomerang you instead because you are in spell range, and spear boomerang is what he does if he is not chasing anyone in particular, yet the AI detects a valid target is around.)
2) no, they will not purposely attack ice wall as they are scripted to only target players. That doesn't mean that an attack may have the side effect of destroying them, especially AoE spells/skills. In case for beelzebub as you are mostly asking about it: vampire's gift is specially coded to instantly destroy ice wall, heal the user of vampire's gift equal to the max HP of the ice wall, yet beelzebub is using this spell as a damage dealer but not intentionally using it to target ice wall.
3) evil druid cannot prevent any of those skills, but undead lv 1 naturally resist poison attacks by 50%, so you will reduce acid breath damage by 50%. Undead cannot be poisoned naturally, so you will also escape the effect of 80% poison by acid breath.
Highly comprehensive be-all-end-all monster skill database:
click here
4) Pneuma prevents any physical based attacks that are either forcibly classified (as "always ranged" or any attacks that can be melee/ranged depending on position of you relative to the monster. If you are 4 or more cell away from the user of a physical attack, then it's considered range and thus pneuma can block it. There are exceptions, mainly vampire's gift and earthquake.
5) Stone skin is not a direct increase in defense, but rather functions like assumptio that it's a % reduction/increase at the very end of the damage formula. It will not affect ice pick. Ice pick is hailed as a very strong weapon against beelzebub because of its 200 vit, not because of stone skin. You will not see damage increase against bee with the ice pick as bee is never death and activates stone skin.
there's some mystery elements regarding stone skin however that the above interpretation may not be entirely true beyond level 1 stone skin. You can physically attack a lv 5 stone skin user, yet the damage is not reduced to 0 (as it should be level*20% reduction). However, a lv 6 stone skin allows a monster to *absorb asura strike* as a heal. It's very evident when you do 100k asura damage, and a lv 6 stone skin user heals 20k instead, suggesting that stone skin reduced the attack down to "negative 20%" So there's contradicting evidence. Even iRO wiki is not sure of this, so they only describe what level 1 stone skin do.
6) As long as bee has 6 slaves, regardless of what they are, then bee will not use summon slave. If bee has 5 slaves or less, bee will immediately spawn 10 hell flies in addition to whatever slaves it has. The 3 chonchon and 7 hell flies are just the initial slaves. Additional slaves are always hell flies.
With that in mind, it is impossible to fight beelzebub with only chonchons as slaves. The most ideal slave count is 3 chonchon and 3 hell fly at best (this is disregarding the practicality aspect).
7) his cast tie is also 0 second. If you can disrupt a theoretically disruptable cast that is 0 seconds in cast time, turn yourself in to the GM now for insane hacking abilities.
8) a much tougher question than you think which is not embarassing at all. Because you see the monster mapflag allows it to change target until quite a few circumstances, I actually don't know the precise condition to change target. I can only tell you what factors come into play:
-distance between current target, and player who is attacking
-distance between current target, and the next closest player
-if the mob is capable to change to "angry mdoe" (refer to galion)
-if the mob used a skill that causes it to move or change location (such as teleport, body relocation, run, back slide)
In most bee setup, the tank is not attacking back at all, yet bee should not change target to chase after the long range attackers, even though in theory they should do so. A factor of 'distance' comes into play, and it discouraged bee to change target. What exact cell range makes it so bee refuses to change target, this I do not know.
If you want to attract hell fly away from the tank and is failing to do so, consider standing closer then attack the fly again. Be careful that when a monster recalls all of its slave, all of its slave will default the master's target to be their target.
9) see the comprehensive monster skill database
10) wizards break lex aeterna, which coupled with an ice pick attack, is considered an inferior way for damage dealing. Stone skin activates way too late for it to be abusable factor even if bee takes an additional 100% bonus magic damage when lv 5 stone skin is turned on. No status effect works on the flies. LoV and SG are both negated by the flies' element, and MS is too unreliable as a way to hit-lock the flies who got a very short animation delay. Heaven's drive only hits 5 times which is also too short to force the flies into animation delay. Spamming to this degree makes them rely extensively on SP which Bee happily burns off with wide soul drain, thus they are resorted to ganbantein/safety wall most of the time only. You'll need to burn off too many sp recovery item then.
i'd really appreciate each 
all the other questions can only be answered by experience tho