then all cards on the field are destroyed unless the union monster has an effect that says that it can be destroyed instead of the equipped monster
Flint is sent to the graveyard
Relinquished's equipped monster is destroyed if Relinquished's ATK is lower than the foe's attacking monster's ATK. If Relinquished is in Defense position, its equipped monster is destroyed if its DEF is less than the attacking monster's ATK. Either way, Relinquished remains on the field.
If an equipped monster finds itself no longer equipped to a monster for any reason, then it is sent to the graveyard.
'Ring of Magnetism' specifically says that it can only be equipped to a monster on your side of the field. If a player takes control of a monster equipped with 'Ring of Magnetism', then the Equip Spell card will be destroyed because it is equipping an illegal target.
It is either a monster that can function as an Equip card through its own effect, such as Union monsters, or, it is a monster forcibly equipped by another card's effect such as Relinquished's ignition effect.
Ignoring things like Relinquished or Destiny Hero Plasma, Pitch-Dark Dragon can only Union itself with a Dark Blade monster you control.
If a monster becomes an Equip card, it loses all of its effects, unless specified otherwise. So what I mean is a Union Monster will then apply its union effects (as long as it was equipped by unioning, not by another method), but if a monster is stolen by Relinquished or equipped to Mermaid Archer, the monster's effect is lost. Spined Gillman's continuous effect to boost your fish, sea-serpent and aqua-type monsters, will therefore not be applied.
"Shocktopus" goes into your Spell/Trap card zone and is considered equipped to opponent's monster.
Any Equip card that doesn't specifically say otherwise, can be equipped to your opponent's monsters. Even if that would be disadvantageous to you, it still can be done. It also means if a monster is equipped with Axe of Despair, it still remains equipped with it even if control of the monster changed.
The new ATK (minus the 500 for trap jammer) would be used in damage calculation.
No, only monsters who specifically say they do, gain such a bonus. Relinquished for example, is one. It gains a bonus depending on what it has equipped, as does Destiny Hero Plasma. Dragunity monsters do not, unless one does actually say so.