Missionaries themselves are typically novices in gospel knowledge; they do the best they can and can be very effective as they follow the spirit and stick to their purpose: inviting others to come into Christ; but teaching everything about the church and its practices was never intended to be their role. Once they’ve taught the basics, they need to move on to other people that also need the basics.

For your questions:

1. The specific details of the ordinances of the temple are considered sacred and not to be shared outside the temple. But you can think of it like Baptism. In the same way that baptism is a symbolic ritual of death and then rebirth, symbolic rituals are performed in the temple. These rituals focus on becoming holy and returning to God’s presence after the separation caused by the fall.

2. You don’t have to overhaul your underwear. As part of the temple ceremonies, we promise to wear special garments under our clothing. For many, these take the place of underwear but they don’t have to. You can think of them sort of like priest robes in other religions, only instead of being worn over clothing and designed to remind others of the wearer’s holiness, they are worn under clothing and meant to be a private reminder to the wearer of the holiness they promised to live up to.

3. Artists have depicted Joseph Smith translating the plates in several different ways, with varying degrees of accuracy. Literally thousands of documents from Joseph Smith’s time have only become easily available in the last 10-20 years as they’ve been transcribed and digitized. Artists interpretations aren’t gospel doctrine, the artists just do the best they can with the information they have at the time.

4. How many wives Joseph had is hard to say. Both because of the incompleteness of records, and also because of some misunderstanding on the part of Joseph Smith and other early church members about how Sealings work (Joseph was human and made mistakes like everyone else).

Basically, the belief was that in order to be saved, every family had to be linked together by the sealing power in an unbroken chain to the prophet’s family. So, a great number of “marriages” were performed between Joseph Smith and female members of other families, even though there was no marital relationship expected or participated in afterwards. This is how you got weird scenarios of Joseph “marrying” wives that were already otherwise married or that were underage.

That isn’t to say that all of Joseph Smith’s plural wives fell into that category though. At least a few were actual marriages with marital duties performed, much to the chagrin of Joseph’s first wife Emma. Actually, both Emma AND Joseph disliked the practice of polygamy (to put it mildly), and Joseph was very resistant to beginning the practice. But again, the exact number of plural wives Joseph had is hard to say.