Today is International Day of Women in Diplomacy! #emdiplomacy was by no means an all male affaire. Women played a central role not only in mainting contacts to the queen's court and other female actors. They could also directly take part in negotiations, as the example of the Ladies' Peace of Cambrai (1529) shows. Here Margaret of Austria and Louise of Savoy negotiated for the Emperor and the king of France respectively.
If you want to know more, have a look at the #handbook article by Carolyn James who talks about female diplomatic actors.
I find it strange that royal marriages are often seen as the CONCLUSION of a treaty/diplomatic process. Surely marrying one's daughter or sister to another king or prince is the BEGINNING of having a close ally permanently placed at the heart of a foreign court - with direct, privy access.
That's true. In general, a treaty might be the conclusion of the negotiations leading up to it, but it's never the end. It's often the beginning of an alliance or a relation on the basis of peace instead of war. In case of marriage treaties it's even closer.