Why is it "muito'' boa instead of "muita'' boa?
Muito, working as very, is always translated as "muito". (It is an intensifier, not a quantifier)
And how are we to infer this?
"Muito" here is an adverb. Adverbs never change!
I was marked wrong for omitting the second “é”. Is this actually wrong in Portuguese or was DL just being pedantic because the “is” was included twice in the English sentence?
You can omit the second one.