Congrats on the break through.
Regarding making any arrangements of any kind to see visiting relatives around or at the time of the reunion, I have a hard time seeing how this could be done without involving others in the boundary setting, and need for same which is happening between you, LordL and his parents. That might make sense if you and LordL had decided to sever ties with his parents, but wished to remain in contact with other relatives. Since the case sounds more like "we are engaged in boundary setting with LordL's parents, and still want them in our lives but are working on dynamics." - that seems like something which would be awkward and uncomfortable for others to be made aware of, and effectively asked to follow along with by scheduling separate visiting time with you and LordL.
If I were asked to meet with a couple separate from a large family gathering with these circumstances, I might well feel supportive, though of who and why would depend on what I knew of the situation, and the individuals - I *know* I would feel uncomfortable with being made a party to the "discipline of setting boundaries" between other family members by agreeing to socialize separately.
I would suggest that if you and LordL continue to decline to attend the reunion, take it for what it is - MIL has put you in the position of either allowing her to do as she wishes, in total disregard of your feelings on matters relating to you two (shower/toast/whatever) or missing out on a group family reunion and chance to see relatives. You did not create the issue, and it is possible for you to "fix" the issue this time, but standing firm will provide MIL consequences for her actions. While you and LordL will miss seeing relatives, I would still not try to arrange separate get togethers as being too likely to spread and grow drama/discomfort, and risk sending MIL a message of spiteful retaliation rather than her own actions having consequences she could avoid.