I ordinarily avoid relligious discussions on this forum like I would the plague. Maybe I should again. But I will make a couple of comments anyway.
Our first article of faith is that we believe in God the Eternal Father, his son, Jesus Christ, and in the Holy Ghost. Another article is, We believe that through the Atonement of Christ, all mankind may be saved, by obedience to the laws and ordinances of the Gospel. (This often sparks the old grace vs. works debate. We teach that it by grace that we are saved, after all we can do. Not going to debate that here.) Another article of faith says that we believe in being honest, true, chaste, benevolent, and in doing good to all men. As I have said, our real beliefs are readily available at
https://www.churchofjesuschrist.org. If what you are being taught about us does not match that, then it is in error.
We do not believe that you can pray your ancestors into heaven. We do believe that certain earthly ordinances are necessary, and we do provide those vicariously for the dead. The deceased person then has the option of accepting that work. It's part of the effort to make sure that everyone who wants the opportunity receives it. Nobody gets left out because of where and when they were born.
We don't have magic underwear. Those who have received the temple endowment do wear special garments as a reminder and emblem of sacred promises we have made to be charitable, faithful to our spouses, and honest in our dealings, etc. It's not so different from other religious emblems, such as the Jewish yamulka or the Catholic clerical collar. The difference is that we wear ours under our street wear.
There is a boatload of anti-Mormon nonsense available. I've read quite a bit of it. It's typically full of distortions, half truths, and outright lies. I suggest that you not waste your time with it, when our true doctrines are so readily available. If you think we're right, you're welcome to join us. If you don't, then we wish you well and will do what we can to help you on your journey.
