Okay then.
In a melee or ranged attack, you roll high on a d20 against a static defence
Actually this is one of the games biggest misconceptions, the defense isnt really static, the defender can roll a d20 too, and that value becomes his base AC for the round. Its not actually used all that often, but you can do it.
but get their via unnecessarily weird subtraction algorithm.
Sounds extremely hard to grasp when you are 6. Any teenager or adult should have no problem adjusting the values tho.
In a magical attack, the defender rolls against a completely static number, except if the defender has magic resistance in which case the attacker also rolls but this time with a d100 against a d20.
Wat? no.
MR is a static value, you dont roll it against anything but your own MR. Also tons of spells dont allow for MR, or dont allow for TS, or dont allow for any, or can ask for STR rolls, etc.
Why have two unrelated, layered mechanics to resolve the magic attack
Because the second one is a extremely rare defense, that is always rolled, even against beneficial effects or even magical items.
and why is the attacker rolling against a defence on the ranged and melee attacks but the defender rolling against an attack on the (standard) spell defence mechanic?
As i said, it doesnt necesarily need to be the case against attacks. Against magic the caster merely summon forces that are largely outside of his control, its up to the one affected to fight them off.
In non-weapon proficiency checks, the player rolls low on a d20 against a static number that may be adjusted by the DM at his discretion (with no general guidelines on what kinds of adjustments should apply when)except with thief skills in which case he rolls on a d100. Why is the roll low rather than high, and why use different dice?
Why not, rolling different values and aiming for different numbers brings variety to the main gameplay element of the game.
Thief skills allow progression and start low. NWPs only allow progression by spending extra slots, and the base chance = your ability score. Why the difference?
Because thief skills are class based, they grow as long as the character takes levels in the rogue class. Also thief skills are modified by your ability scores as well. NWP are outside of the scope of your class.
And what kind of sense does the NWP mechanic make?
Plenty, makes it clear that its completely divorced from your profession or chosen path. Its the knowledge you get while living your lyfe.
To resolve a fireball attack against, say, a group of three drow, you need:
(1) A roll on d100 against a number from the monster description for each drow, to beat magic resistance
(2) A roll on d20 against a number from the monster description for each where you successfully beat the magic resistance check
(3) A roll of nd6 where n is your character level, which you then need to sum, and divide by 2 for each drow who succeeded in the saving throw
All this ordeal takes like half a minute.
Don't you think this is rather a lot of rolling, comparing, adding, and dividing for resolving a simple AoE spell attack?
No, its simple math.
True or false: characters can opt to use NWP slots to buy thief skills?
That rule doesnt exist as far as i know. But i never looked into it. May have heard something about it at some point.
I do know there are non weapon proficiencies that give bonuses to thieving skills, like being a locksmith for example.
A character with STR 18/25 has a Bend Bars chance of 25%. He notices his jail cell's bars are bronze rather than iron. How does that affect his Bend Bars chance?
I dont know? whats the state of the bars? is there a structural weakpoint? i roll engineering to see if i can improve my chances, maybe the bars are greasy, that would make it almost impossible regardless of str. Will i be attempting it in the middle of a fight?
Chapter and verse plz, this is a common situation so "the DM will asspull the adjustment" is not an acceptable answer.
In PnP is the only real acceptable answer. Rules dont matter, the power of the DM is absolute.
A character with STR 18/25 needs to push a heavy boulder over a cliffside. How is his chance of success determined?
Str check i guess, or compare his max carry weight to the weight of the boulder. What is a heavy boulder? just how heavy is it? how big is it? is there an inclination or is the surface completely flat? What about the weather? has the character exerted himself recently?
Sure.