You mention God's contingency plan for those who never had the chance to hear The Message, and for Mormons, baptisms for the dead is the contingency plan. The Bible is quite clear that those who don't believe and aren't baptized will be damned, with no qualifying exceptions. But how could a God who is just and fair allow so many billions of people to live on the earth without a chance to believe and be baptized? Baptism for the dead is the answer. Paul mentions it while teaching the universality of the resurrection; Peter said that Jesus taught the spirits in prison between the time of his crucifixion and resurrection. The practice is completely in harmony with Biblical doctrines.

Mormons also don't believe in infant baptism. It's an ordinance intended for those who are capable of believing and repenting; the minimum age has been set at eight years old. Certainly an infant isn't capable of consenting to baptism and all that comes with it, but that doesn't stop a lot of people from making that choice for their own children.

I wouldn't be offended if someone declared me to be a member of their church, posthumously or not. I don't think other churches have the divine authority to make binding priesthood ordinances, so my reaction to their attempts to include me would range from amusement to indifference.