Yes. Insulin, including humulin, (insulin) can be safely taken when breastfeeding.
Yes. It is safe.
While it has never. Actually been studied or approved for use in pregnancy or breastfeeding, it has been used in those situations by millions of women without reported problems. Check w/your doctors and get their opinion.
Yes. Yes it is safe.
Probably. It's still considered a category c drug which means it can be used if the benefits outweigh the risks. Most high risk pregnancy people feel that it is ok. Talk to your endocrinologist.