"+5" on an item name means 5 more damage per attack, not per hit. e.g. if you only attack once but the attack does 3 hits, you'd get +5 damage total, not +15, because the bonus is per attack. (There are some effects that let you attack more than once in a turn, like haste, at which point you'd get the bonus more than once).
"-5" on an item name means you take 5 less damage from each monster attack. So you're right: if you got a -5 item on floor 1, the earliest a monster could damage you is a strong monster on floor 3 (3x3 damage).
"Attack" and "Hit" are hopefully used consistently in the rules, but combat is broken down like this:
Turn → Attack → Hit
Depending on the effects you have, you can attack more than once on your turn, and each attack can hit more than once.
Thanks for pointing out the invulnerability combination, as well. I've updated the PDF so that the "Doubles" effect now only works on non-6 doubles. I also changed the "Block" effect, which stops all damage from the minimum attack target number. But you can increase this to 6 for strong enemies, with other effects, which is another way to block all strong enemy damage.