How to determine which item/role mechanic overrules another, if they contradict each other?

For the most part, each type of role is resolved in fairly generic but consistent ordering which is documented on the how to play werewolf guide on the site. The most obvious contradiction is protection from death and an ability that kills. In this case you have a "X will happen" (kill) contradicted by a "Stop X from happening" (protection). So all roles that have a protective impact are resolved before roles that have a killing impact.

From the guide:

Redirects (Succubus)
Roleblocks (Direwolf)
Protection visits (Protector, Huntsman, Shaman)
Most visits (Too many to list: all visits not belonging in another category)
Active Item visits (Crossbows, potions etc)
Resolve kills (Killing werewolf (Werewolf/Alphawolf/Shapeshifter), Vampire, Witch, Militia)
Report visits to Stalkers, Harlots, Familiar
Resolve item thefts (from abilities, potions and kills)
Resolve item transfers (that have not been used or stolen)
Swap Identities (Djinn, Shapeshifter)
Report kills to village
Report revives to village

Items that fit into the protection or kill categories will be amalgamated and resolved with role abilities that have similar effects.

Now, relating to your Bloodletter example. Some roles and items can modify the properties of the player. These modifiers are for the most part hidden (although a rarely used role called the Aura Seer is able to see the names of modifiers affecting a player). They are also fairly poorly documented, but each modifier can alter the base property of a player. For example, change their viewed faction from village to werewolf, or reverse their witchcraft status.

Each modifier has a unique initiate property that determines the order to resolve them in, this isn't well documented on the guide yet, so I will copy it here as a reference.

1    - Bloodmark
2    - UnholyEavesdropper
3    - ThreatOfRetribution
4    - AvatarOfRetribution
5    - EnduringSpirit
6    - PossessingSpirit
11   - RiteOfFenrir
13   - BloodSoaked
14   - Lycanthropy
15   - DemonicEssence
16   - HasAPointToMake
17   - ItchyTriggerFinger
18   - CarryingPoison
19   - Paranoia
22   - Lycanthropy
23   - SecondChance
24   - ReincarnationSpell
33   - RunicAura
34   - Spellbound
35   - Whispers
55   - Poisoned
64   - Blessed
65   - Decay
66   - DemonicDarkness
67   - ContagiousBlight
68   - Blighted
69   - Inebriation
70   - Damned
71   - AbysmalProtection
72   - Pollymorphed
73   - SpittingFeathers
74   - ImImmortl
75   - Crippled
76   - PrimedBearTrap
85   - Resilience
88   - Weakness
90   - StrongNeck
91   - PaxDemocracia
96   - Shadow
97   - StrengthOfFenrir
98   - Tenacity
99   - Shadow
100  - MartyrsProtection
999  - Vulnerability