NTJ but you obviously didn't put enough thought into the lights. They stick anywhere, don't have to be hung up with the provided sticky if she does want to hang them on the wall, and you didn't have to just forbid them. You say she wants a lamp. You could have led her to choosing to exchange the LED lights for a lamp.
If your mother continues to interfere in your parenting, start conversations with "Mom, if you aren't going to hear both sides before going off at us we're not going to discuss this."
I can't figure out if your oldest is deliberately causing trouble or just trying to get little sister the things she wishes she had at that age. Whichever it is, you need to learn better ways of guiding both your children. It sounds like you both are punishing little sister for big sister's gifts.
You said he apologized when he came home. Was he out visiting his mom? I wonder because it seems like he talked with her and she supported your side of things.
I don't think he's a Mama's boy as others are saying . He just seems to have a close relationship with her. That's not a bad thing. However, he may be wrongly guessing at her needs. A lot of young adults think their parents won't have a fulfilling life when the kids move out. It's especially common when a single child has been raised by one parent. Perhaps his mom could reassure him that she will be fine and enjoying life.
If he just wanted to get to know them, that would be great. However, it sounds like he only wants their phone numbers so he can threaten them. That is not okay. I believe he should meet them in person. Then, it's up to them to share their contact information. There is nothing wrong with having friends of the opposite gender. Your husband is jealous and controlling. Does he have any friends there? Maybe you should introduce him to people. Couple's counseling also seems advisable.
I'm an introvert. I know what it's like. Your partner is not an introvert; he's hiding. In the house and in "substances." You're worried about how introverted he will be in his old age, but you should be worried about how addicted he will be by then. If he even survives. He needs help quickly.