I have a question about intimidate specifically as it applies to Chella. In a number of circumstances, Chella will trigger intimidate with greater strength than when the challenge was decided. Is the strength that can be knelt determined by the difference in strength when the challenge is decided, or when intimidate is resolved?

It's calculated when the challenge is decided.

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Intimidate
Intimidate is a challenge resolution keyword. After a player wins a challenge as the attacking player in which he or she controls a character with the intimidate keyword, that player may choose and kneel a character, controlled by the losing opponent, with equal or lower STR than the amount of STR by which the challenge was won . (The chosen character is not required to be participating in the challenge.)
• No more than one instance of intimidate can successfully resolve during each challenge.