Missionaries do go through the temple before they become missionaries. I totally agree with doing research before joining a religion though, and the fact that not all temple stuff can be found in the Book of Mormon (or other religious texts).

You are correct, they do go through the temple before their missions. But very little is revealed to them before/during their missions. As they come home from missions, get married, sealed, and continue in church duties, it becomes "Pay2Play" as new tokens are "revealed" to them since the only way to continue further is by making sure you are paying your tithing.

I also agree with your second point, even if you choose to believe a religion, please educate yourself from all sources and do some critical thinking for yourself.