I agree with the posters who say stay away from the reunion and that you can't have it both ways. i would not go before or after either--stay away from the area. Either you suck it up and go to the reunion and let mil have her shower/toast (which is not what you two want and will only show mil that if she harps long enough, you two will give in and she can do what she wants) or you tell her you will not be there.
If you do go and try to have a get together without mil finding out, it will probably backfire on you. If relatives find out you are in the area and not at the reunion, they will surely ask questions. Trying to plan a brunch with them during or even after the reunion will cause them to ask why you aren't at the reunion or for them to call mil and ask what is going on. Then you will really have a mess on your hands. If you are not in the area, then there can be no guessing as to what is going on, why you are snubbing the family, etc.
Better to tell mil "We will not be at the reunion. we asked you to not have a shower and lordl asked you to wait on sending the invitations, but you did anyway. We do not want the reunion to be about us, especially since not everyone who is attending is invited to the wedding. So, we will not be attending."
While it is sad that LordL won't be able to see some relatives at the reunion, the two of you should make plans to see them another time or if they are invited to the wedding, you will get to see them then.