Lawful and Chaotic while using spells, this question was asked a while back, not sure if ull payed attention.
Only heal spells are affected by alignment.

Why cant my lvl 45 mage hit some chars I thought MR was a % of dam taken away from the original damage, but it has come to my attention that some spells miss (erupt, para, ebolt, tornado).
Characters can avoid spell damage based upon their ER (ER is explained next)

What exactly is ER and how does it factor in?
ER is your percentage chance to avoid a ranged attack. This includes spells and arrows. If you avoid a ranged attack you will take no damage from it. It will not affect melee attacks. ER (Evade Rating) is determined based upon class, level, and current dexterity. There is no random gain in ER at level up time. Knights have the best ER.

Is the damage you receive from monsters proportional to the lvl at all. (not counting er ac or dex just lvl). Example if I had a –50AC, 10ER knight at level 40 and a –50AC ER10, level 30 knight would I receive the same damage from a mop or does level also matter (or is it just my eyes that sees my hp drop faster because of less hp).
A player’s level does not factor in to damage received from monsters. That is determined by other means.

Where do drops go after your bugbears are full?
They disappear from the game.

Does ghast still drop summon?
Yes.

True/False: when you shoot an arrow at an enemy, the amount of damage inflicted is based upon the enchantment of the weapon, the type of arrow, and your DEX.
False. Also includes any damage bonuses to the bow, but not the bows base damage. Also if the opponent has other special attributes that damage a weapon used against them, then it will reduce the damage by half.

True/False: When you swing or shoot at an enemy, the probability of you landing that hit is based Only on this list: your dex, your level, enemy ER and enemy AC.
False. It is also based upon weapon enchantment (a 6 weapon has a 3 to hit), and weapon hit bonuses, and your strength. It is also based upon your encumberance level. If you are less encumbered, you have a better chance to hit.

True/False: When you cast damage inflicting spells at an enemy, you will always hit (even if the damage inflicted is zero) and the amount of damage inflicted is based ONLY on this list: your SP, enemy MR.
False. The target can evade the spell based upon their ER.

Is the damage/hit formulas for PvP the same as for killing monsters?
Monsters do not have ER.

Is the difference in HP between one mage with 19 CON and 16 INT versus a mage with 16 CON and 18 INT made up in the difference between each mage’s spellpower?
Higher CON will give the mage higher hitpoints, higher INT will allow the mage to do higher spell damage. The effect of spell power on spell damage differs per spell. There is no fixed increase in damage based on spell power.

Does De-levelling and re-levelling have a negative impact on a roll? Is there a pre-determined range for this scenario or is it completely random?
It’s random. You could potentially lose more when de-levelling, because the same type of calculation is made when de-levelling.