Do some research on prayer. There’s quite a lot of research that’s been done on it. (Plus, my own experiences.)
If I am dead, I am in heaven, hell or purgatory.
Their ceremonies are going to do nothing to me. Let them do what they want to me. I’m going to be, Lord willing, praising Him and won’t care at all.
I'm not sure what prayer has to do with it. Either Mormons who baptise Catholics thinking they are in hell and won't see heaven without this baptism are right or they are wrong. I think they're wrong.
If they want to waste time baptising some "proxy" on behalf of one of my dead relatives, I wouldn't care. Mormonism isn't true, so it's not going to have any impact.