Replying with "I love you too" is perfectly reasonable because it gets the point across and no one is likely to misunderstand that "too" is meant to reflect the sentiment back to the first speaker. If you are uncomfortable with the ambiguity of the meaning of "too" in this context, I suggest saying "And I love you."
Yes, you may start a sentence with "and", as per http://blog.oxforddictionaries.com/2012/01/can-i-start-a-sentence-with-a-conjunction/
Edit: As noted in the comments, the question relates to the ambiguity of the pronouns rather than the interpretation of "too". But the suggested answer of "And I love you" is still suggested because by removing "too" it avoids the potential for misunderstanding the pronoun.