What do I do?

Me and my husband have been living with MIL for almost a year due to financial reasons. I am 15 days post partum and she’s arranged an after birth party for me. This is traditional to our culture but it is usually done 40 days after the birth to give the woman time to heal and adjust to her new life. When MIL told me about the celebration I told her I wasn’t ready and concerned about my newborn. I don’t want her around 50+ people with such a delicate immune system , especially in this season. MIL told me it was too late to cancel as she’s already paid a deposit for the venue and food and paid for a bunch of other stuff for the celebration. She thought I was going to be over the moon but I’m quite disappointed. Also note she wanted to do the party this early because she has many friends who would come to visit if we don’t. To avoid having unexpected guests over every day she’s doing this. I’m trying to convince my husband to stay with our newborn since it’s a woman’s only party he’ll just stay home. We have another 3 year old who he’ll be staying with but I want him to also keep the baby. He will have his uncle to help and his wife is coming to the party too and leaving their kids with him. They will all stay together at MIL house. My proposal is he stays with both our kids and I just make an appearance at the party and come home after an hour. He says it’s irresponsible and not a good idea for mother to be separated from baby so early on. He also doesn’t trust he’ll be able to care for baby. MIL suggested another option. It’s for our newborn to stay in a room with a cousin at the venue. The cousin ( a 14 yr old) would watch the baby. This way she’s away from all the other people and the risk of her catching something is lower. She will be placed in her car seat the entire duration. I am not comfortable with this as she hates her car seat and honestly I don’t think she’s very comfortable in it either. I’m also worried about the increased risk of SIDS. MIL says she’ll keep checking on the baby but I don’t trust her with my newborn as she routinely overheats her. I’ve spoken to her time and time again about overheating but she insists she knows better than my midwives and doctors because she’s had 4 kids. Now I’m worried about overheating and SIDS from breathing difficulties due to being placed in a car seat. Can somebody advise on what I can do? How can I convince husband to watch her for the 1 hour I’ll be gone? Am I overreacting/ letting my anxiety get the best of me? Will she be okay sat in a car seat for 1 hour ? I will end up just holding her the entire duration because of this anxiety. I don’t want people near my baby though and keeping her on me will expose her to them. I’m really not ready for this party but it’s in a few hours.