We all know that there is a custom on Shavuos to eat dairy. One of the reasons given is based on a Medrash that When Moshe Rabbenu went up to Heaven to receive the Torah the angels complained and said "How could this man of flesh and blood be worthy to take the Torah?" Hashem answered that when the Malachim (angels) came to Abraham they themselves did not keep the Torah, as they ate meat and milk together. The angels accepted this answer (among others) and hence on Shavuos we eat dairy.
With that introduction here are the questions.
The Torah law only does not allow cooking milk and meat together so how were the Malachim not keeping the Torah?
If it was cooked together, how was Abraham allowed to feed them this mixture? Abraham kept the whole Torah, and milk and meat are Assur Bhanah (one can't have benefit from the mixture). This prohibition applies even if one wants to feed it to a non-Jew?