“When it’s raining outside to you eat and sleep in the sukkah or you go inside? I don’t know, but guessing that you go inside and rely on the retroactive hetter, even though the Rama says we should be machmir.”
The Rema says the exact opposite of what your claiming he does in siman 639. I’m not sure if you’re lying on purpose or by accident. The only thing he says that you could even somewhat twist into claiming he agrees with you is with regards to the first night of Sukkos where you have to eat a kazayis even in the rain without a lesheiv b’sukkah. Even that’s just a hachraah minhag as there are rishonim who hold you would be patur on the first night as well, so for someone to go inside even on the first night would not be mevatel a mitzvah, unlike the case of not sleeping in the sukkah when you would be perfectly able.
It’s a straight Mishnah not to eat in the Sukkah in the rain (I assume you didn’t know this since you called it a retroactive heter). I have no clue what Chabad’s heter is to argue on the Mishnah here.
Username: You can’t start bringing proofs down from poskim who talk about the cold when that’s never been your argument. Nobody is disputing that heter. What people are disputing is your habit of speaking of a heter as though it’s a chumrah to imply that those keeping the mitzvah are doing a bad thing.
I don’t know where this persecution fantasy you guys have is coming from. Nobody has said we would be OK with non-Lubavitchers having this custom. I would be just as critical of any other group for having this shittah. Chabad happens to be the ones still doing it and talking about it.