• Without Target:

1. The ability is summoned from your hand;
2. The effect is resolved;
3. The ability goes to its owner's graveyard.

• With Target:
  1. The "Target" condition must be met (targeted Hero / Ally / Ability / Item or Location);
  2. The ability is summoned from your hand;
  3. The effect is resolved;
  4. The ability goes to its owner's graveyard.

1. The "Target" condition must be met (targeted Hero / Ally / Item);
2. The ability is summoned from your hand and attached to a Hero, Ally, or Item;
3. By default, if the attachment ability is destroyed or if the ally it was attached to is killed or the item is destroyed, the attachment is also removed and goes to its owner's graveyard;
4. To date (August 2024), there is no ability that can be attached to another ability or location.

1. The ability is summoned from your hand;
2. It is placed in the Support Area;
3. And goes to its owner's graveyard at the last turn of its duration;
4. There is no known duration effect bonus or penalty as of July 2024.

• Can an activated ability ever be considered a passive ability?
  

No. Even if the activated ability has an ongoing effect that grants a keyword passive ability to itself or other cards, the activated ability is still an activated ability and not a passive ability.

1. e.g. Moonstalker has an activated ability that can grant himself and friendly allies the stealth keyword passive ability. When the ability has been activated, there are now two different abilities in effect on Moonstalker - the activated one that is granting the passive ability and the passive ability that was granted.
   
2. e.g. Unaxio Bannerman has an activated ability that can grant another friendly ally the haste keyword passive ability. When that ability has been activated, the affected ally will not lose haste if Layarian Diplomat's ability is used on Unaxio Bannerman. You would need to use Layarian Diplomat on the affected ally.
   

• Is an "When X is summoned ..." ability considered a peripheral ability or passive ability?
  

It is a passive ability because it is in play at that point (because it has been summoned) and its effect triggers once in response to it successfully entering play via summoning.

• Is losing passive abilities a one-time thing or is it ongoing?
  

When a card effect states that an ally/hero loses all passive abilities, it is always an ongoing loss, which means that current passive abilities and any that might later be granted while there is still an ongoing loss effect will be treated as though they do not exist.

1. e.g. If an ally has stealth and Layarian Diplomat is summoned to target that ally then it will effectively have no stealth while being acted upon by LD's effect. If that ally later has/gains ambush, then Layarian Diplomat will make it as if that ally does not have ambush either. Once Layarian Diplomat's effect has ended (by the specified duration or LD leaving play), the stealth and ambush and whatever other passive abilities would have been there without LD's interference will be usable again.
   

• Is a negative effect a passive ability?
  

No. A negative effect is a state of being, which is why we say "target ally is poisoned". A passive ability is something you have, which is why we say "target ally has/gains stealth".
The confusion seems to stem from negative effects usually having bold keywords, and keyword passive abilities also being written in bold - the use of bold is always to show these keywords have a defined meaning beyond what might be understood usually from the word alone (some games call these keywords loaded keywords).

• If an activated ability is having an ongoing effect, does it continue if the card with that ability leaves play?
  

No. The effect ends, since the card is not in play to affect the game any longer.

1. e.g. Using the activated ability of Gold-Laced Shield with its last durability will make it leave play (destroyed due to durability becoming zero) and so the effect will be rendered pointless even though the specified duration was for longer.
   

Note that permanent stat changes are not considered ongoing effects. The effect has completed once the change has been made. If you are unsure, look for a duration specified for the effect (ongoing effects within activated abilities always have some form of wording to specify their duration).

1. e.g. "Target ally gains +1 base attack." is not an ongoing effect.
   

• Is Sustain a passive ability?
  

No. Sustain is not a passive ability. It's a cost to be paid in full at the point that card would be readied at end of your turn. This cost can't be prevented or reduced, if you have the capacity to pay it; if the Sustain cost can't be paid in full, the card with Sustain is killed/destroyed.