Let me see if I've got this straight;
1. MIL proposes that she hold a shower for you at the reunion. You discuss it and *as a couple* decide for many reasons (not the least of which is that it is rude, rude, rude) you don't want that. However an informal toast is fine.
2. You inform MIL of your decision but then realise that she is going ahead and planning it anyway.
3. You re-inform MIL of your decision and she lies to your face about not having sent invitations. When called on this she gets her husband to apologise for her and then sends a two word non-apology to you on facebook. Except even while sending that 'apology' she is still working on LordL to get exactly what she wants.
4. Now LordL wants you to give her exactly what she wants because she has made the 'compromise' of letting FIL have the nominal credit of organising the exact party that you *as a couple* said no to in the first place. Since apparently LordL has the awareness of a new born kitten when it comes to his parents, he thinks that this is a genuine sacrifice rather than naked selfishness dressed in a bow, and so you're now the bad guy for not accepting MIL's noble compromise.
Please, as one former MIL-victim to another, do NOT go to this shower. I agree with earlier posters that a wedge is being driven between you and LordL over this. However, I think that playing nice and going to the reunion will just change it from being a visible wedge as it is now, to being an invisible wedge, as you sit, fake smile plastered on, seething inside as MIL gets to play Queen for a Day. Sure, LordL will be happy and you will no longer be arguing but personally that would make me lose a lot of respect for my future partner if that was his preference.
Honestly, I agree that this is a relationship issue now and etiquette is irrelevant. How you deal with it completely depends on your type of relationship and how you communicate. Personally, - and the disclaimer is that this is not relationship advice, it's just how MY relationship works - I would be spelling it all out in one last "Come to Deity" talk consisting of outlining very clearly as above EXACTLY how things have gone down, refusing to accept ANY responsibility for LordL's discomfort and so on. Because honestly if he is going to agree to something with you, say he wants help enforcing boundaries with his mom, and then stab you in the back the instant mom is even a little bit unhappy, then ... well then you have to decide if you can deal with a lifetime of that. At the moment LordL doesn't have your back, and that's not okay imo.