1. Yes. She gets twice the % if the gift is her favorite. One tip with gifts is to focus on 4 girls with different preferred gifts at a time.

To illustrate the first answer: X girl's favorite gift is cake. If you gave her a large cake (+3% base), she'll get +6% instead of the +3% from a single gift, maximizing it because it's her favorite. If you give her any other large present (toy, book, or jewel), she'll only get the base +3% and that's not efficient.

2. Evolution doubles the HP/ATK/DEF stat bonus depending on what her current % is. The % itself doesn't change. For example, if a girl is at 98% affection status upon evolving, she stays at 98% after evolution, but the HP/ATK/DEF stat bonus that goes with 98% affection status will be doubled. The final stat bonuses will be the same ultimately.