Read the Ikoria Mechanics page on the Mothership but the following still aren't clear to me:
If your maindeck post-sideboard breaks the Companion conditions (e.g. you board an Eidolon of Rhetoric into your Lurrus of the Dream-Den-Companion deck), can you still cast your Companion from outside the game?
If you have more than 1 applicable Companion card in your sideboard, can you cast all of them from outside the game in the same game? What if they both have the same name (e.g. 2 Lurrus of the Dream-Den)?
100.6a. Tournaments usually consist of a series of matches. A two-player match usually involves playing until one player has won two games. A multiplayer match usually consists of only one game.
100.4. Each player may also have a sideboard, which is a group of additional cards the player may use to modify their deck between games of a match.
You can have up to one chosen companion for each game. (...) Just before the game begins, reveal your chosen companion to all players. Once during the game, you may cast your chosen companion from your sideboard. Doing so follows all the normal rules for casting a creature spell, so do so only during your main phase.
(...)
Your companion's deckbuilding rule applies only to your starting deck, which is the deck you begin play with each game.
So, if you're sideboarding, you are in the second or later game in a match. Everything that was said to us about Companions talk about a game, i.e., each individual play, not the whole match.
Your deck needs to follow the rules at the beginning of each game, i.e., after you're done sideboarding. And you can only have up to one companion, not more, not even multiple copies of the same companion.
Your deck needs to follow the rules at the beginning of each game, i.e., after you're done sideboarding. And you can only have up to one companion, not more, not even multiple copies of the same companion.
To be clear, a player can have cards with a companion ability in their deck, in the same way as though they had no companion abilities. However, a player can choose only one card with a companion ability as their companion for a game, not several even if they have the same name. That card has to start outside the game (and therefore outside the player's deck).
100.6a. Tournaments usually consist of a series of matches. A two-player match usually involves playing until one player has won two games. A multiplayer match usually consists of only one game.
100.4. Each player may also have a sideboard, which is a group of additional cards the player may use to modify their deck between games of a match.
And from the article, https://magic.wizards.com/en/articles/archive/feature/ikoria-lair-behemoths-mechanics-2020-04-02
You can have up to one chosen companion for each game. (...) Just before the game begins, reveal your chosen companion to all players. Once during the game, you may cast your chosen companion from your sideboard. Doing so follows all the normal rules for casting a creature spell, so do so only during your main phase.
(...)
Your companion's deckbuilding rule applies only to your starting deck, which is the deck you begin play with each game.
So, if you're sideboarding, you are in the second or later game in a match. Everything that was said to us about Companions talk about a game, i.e., each individual play, not the whole match.
Your deck needs to follow the rules at the beginning of each game, i.e., after you're done sideboarding. And you can only have up to one companion, not more, not even multiple copies of the same companion.