Single seating: True
Nothing holding you in: False
It has a T-bar that comes down over your lap. This is NOT a looping roller coaster. It's "old school" metal tracks, so kind of bumpy (in comparison, RnRC is a much smoother track). The seating is single file. Each car has four seats, they are configured in 2 car trains (so 8 people total can ride in one "train" if that makes sense).
It is a very dark ride. For the most part, you can see very little. I told my nephew once, it's like flying thru space. He was petrified to try it but once he did it, it was his hands down favorite ride (he was 9 at the time).
The height requirement on Space Mtn is 44". My four year old could have ridden it on our last trip in October (he was 4-1/2) but we chose not to let him because he still seemed so small to me (he was only about 35lbs). He is just over 40lbs now, and is begging to go on it, so we are going to let him ride it on our next trip (he will be almost five). He is really excited about it.
eta: Oh btw, on our last trip we were still debating on whether to let him ride or not. My parents were with us so we let them watch ds while dh and I went on it together just to see for ourselves how rough it was and make a final decision for that trip whether we were going to allow him to ride it or not. After riding it once, we decided at that time, it was too rough a ride but more than that there are some sudden drops (not huge ones though, but for a 4yr old might seem dramatic) in the dark and I was concerned because I wouldn't be able to touch him he might get freaked out. The restraint is built so it will hold a smaller body well. I did see several young children riding it that looked like they were about his size. My concern at the time was more about how scared he would be than his physical safety after seeing the other smaller kids riding it.
I also wanted to add, he is normally a bit of a dare devil. This is the only ride he hasn't ridden yet that he meets the height requirement on.
hth!