In order to go to the temple, one must be a member of the LDS church and worthy to go. I know there is a lot of discussion out there that parents aren't allowed to see their only daughters get married in the temple, etc. My sister's husband's family is not LDS, and we were in that same situation. I do think the matter needs to be handled with a good deal of prayer. My parents met with his parents, and they talked about the fact that they couldn't be at the temple marriage ceremony, my dad explained about the temple, the covenants my sister and their son would be making, and why. Also that is a commandment from God, and we are following his commandments. At that time, I hadn't gone to the temple either, so I spent the time with them during the wedding ceremony in a waiting room in the temple. They had many questions, and I was able to answer them. The day turned out to be very nice. Last month when my sister and her husband and kids went to visit them, my brother-in-law's father pulled him aside and told him how happy he was that Steve is a member of the LDS church, and that he had married my sister in accordance with its teachings. He could see the happiness in his son's life.
And I do know of a couple of instances where the statement was made "Well, you are not worthy to go to the temple to see your daughter married" and the attitude was rubbed in by this particular person. It created tension between him and his in-laws that has never been resolved.
My father has been a bishop a couple of times, and has dealt with the issue of parents not being able to go to the temple or other members of the family. He says that if the subject is approached in prayer and in love, that the parents do understand.
As far as the ceremony goes, about 20 minutes, average time, I would guess. Depending upon the temple and time of the year, yes, there can be a lot of ceremonies on the same day. How many, I don't know. But the temple workers make the day special for each bride and groom, there is no rush through the temple, nor looking at watches, etc. My parents have jokingly told us that none of us should plan to get married on Valentine's Day (my brother did) or just before Christmas (like my sister did.)