At this point I am perfectly willing to call MIL myself and lay it all out on the line - NO SHOWER. Politely but firmly. In the past I have insisted all communication go through LordL but I am willing to make an exception - thoughts on whether this is a good or bad idea?
I'm not sure it will be an effective idea, unfortunately. My bet will be that if you show up to the reunion, there will be a shower, or a shower equivalent dressed up in slightly different wording.
As hard as it is to put LordL in a difficult emotional position, he can't have it both ways. He can't ignore your needs/boundaries to placate his mother and then expect you to be there to support him while he visits relatives who make him nervous and protect him from the stress MIL causes.
This is a very good point. You can't put your wife through the ringer with your family and not back her up properly, and then expect her to happily act as a shield to make interactions with your family less painful. There's likely a very good reason his relationships
with his family are tense - having his wife there to soothe/protect him isn't going to make those reasons disappear.
On another note - I see the new working as definitely being a shower, because these are invitations to a family reunion, that are being sent to non-family members, solely to invite them to celebrate a forthcoming wedding. You can call it a shower, or an engagement party, but it's not a family reunion at that point.
If your MIL were a normally reasonable person who really, really wanted to have a family shower, then I'd say going along with it would be okay for family happiness. By the sounds of it, this is one in a long string of incidents where your MIL tries to do something unreasonable, you object, she does it anyways, and your fiance is puzzled about why you're so upset, because after all, you did stand up to her by saying no. That's not just an etiquette nightmare, it's marriage dysfunction in the making.
And quite frankly - it's better to make a stink over an unwanted bridal shower than to have to make a stink actually at your wedding when she pulls something else (unwanted guests, maybe?).