#1 decided = came to the conclusion (change o state)
thought = expresses an ongoing state (no change)
#2 Yes, most able. When connected by "and" the adjectives must be in the same form (superlative)
#3 For the husbands, there is no difference, They're all dead. For the mood of the sentence, the mood is one that her deed was incredible. Example:
To kill one husband is terrible, to kill two is unbelievable, to kill three, four, five, well, you get the picture.