[refactor/bugfix] use rule 802.2a where appropriate.

[Grath]

Many effects which relied on getDefendingPlayerId would fail if the attacking creature had been removed from combat before they resolved, in which case the defending player ID would be null. This fixes these issues.

[xenohedron]

802.2a.
Any rule, object, or effect that refers to a "defending player" refers to one specific defending player, not to all of the defending players.
If an ability of an attacking creature refers to a defending player, or a spell or ability refers to both an attacking creature and a defending player, then unless otherwise specified, the defending player it's referring to is the player that creature is attacking, the controller of the planeswalker that creature is attacking, or the protector of the battle that player is attacking.
If that creature is no longer attacking, the defending player it's referring to is the player that creature was attacking before it was removed from combat, the controller of the planeswalker that creature was attacking before it was removed from combat, or the protector of the battle that player was attacking before it was removed from combat.
If a spell or ability could apply to multiple attacking creatures, the appropriate defending player is individually determined for each of those attacking creatures.
If there are multiple defending players that could be chosen, the controller of the spell or ability chooses one.

[JayDi85]

Can you provide some buggy use cases or tests?

[Grath]

Can you provide some buggy use cases or tests?

I'll confirm my suspicions in a sec to write a test for a simpler buggy use case, but I encountered it trying to do [[Norin the Wary]] + [[Blade of Selves]] - Myriad would fail due to null defending player when Norin had already exiled himself before Myriad resolved.

Must have 802.2a rules text here or it's part (about founding of defending player). Can it be a default logic (allowFormer = true)? Looks like rules used it for anything.

Yeah, it's probably possible to default it to true. There's a couple use cases where it's important to be false I think, but most cases it can be true. Lemme look over it a bit further.

[github-actions]

Norin the Wary - (Gatherer) (Scryfall) (EDHREC)

{R}
Legendary Creature — Human Warrior
2/1
When a player casts a spell or a creature attacks, exile Norin the Wary. Return it to the battlefield under its owner's control at the beginning of the next end step.

Blade of Selves - (Gatherer) (Scryfall) (EDHREC)

{2}
Artifact — Equipment
Equipped creature has myriad. (Whenever it attacks, for each opponent other than defending player, you may create a token copy that's tapped and attacking that player or a planeswalker they control. Exile the tokens at end of combat.)
Equip {4}