We had the similar issue and Tridion created a hotfix CME_2013.1.0.88281 for us which enables multivalued fields to be populated even after the first instance. It works with everything apart from keywords and categories. I would suggest not using categories and keywords and then it will work fine.

We also raised request for categories and keywords and it is right now added as a defect which will be added in the next major release. Defect # 88297

We had the similar issue and Tridion created a hotfix CME_2013.1.0.88281 for us which enables multivalued fields to be populated with default value even after the first instance. It works with everything apart from keywords and categories. I would suggest not using categories and keywords and then it will work fine as long as you have the hotfix.

We also raised request for categories and keywords because after the first instance, the default value is not populated if we use category and keywords. It is accepted as a defect which will be added in the next major release. Defect # 88297