The first sentence is just context and does not have a place in our conditional chain. You are correct that the quantifier ELCI should "some" over to LCI. So replace ELCI --> LCI with ELCI -some-> LCI. Still ac(A) is correct. Nice spot!
Right @nye8870, if you just stick a not in the second clause of the dog tail example it's not exactly the negation. Negating the second clause "then you have a tail" would be "then you may not have a tail" or "then you do not necessarily have a tail."
... lesson you referenced is for negating intersection statements
negating conditional statements. It literally ... says "Negating the Conditional Sentence” in ... irrelevant because it’s negating the sufficient condition (just ...