[Aerich]

1) Thac0 - as you surmised, the first number is "natural" thac0 - from class and level. The second number is thac0 modified by strength, racial bonuses, equipment, and magic. High dexterity will modify thac0 if you are using missile weapons - +1 at 16, +2 at 17-18, +3 at 19.

2) That resistance is innate or (rarely) item-related resistance to your attack form. If you look on the record screen and scroll down, you will see a list of the possible resistances. I've noticed that shadows have a little resistance to missile weapons (maybe 10%) as do verbeeg (possibly 25%) and skeletons are 50% resistant to slashing, piercing and missile attacks.