We've got one with a flat bottom rail at the top of the stairs so there is no trip hazard. It's also widely recommended to have a screw attaching one to the wall rather than just the pressure fixed ones I think!
Yes it’s fine to have one however they recommend you have one on your child’s doorframe too x
I don't think they're recommended at the top of stairs as toddlers could climb up and over them or push too hard on it and dislodge it, both of which can result in falling down the stairs. I think that's why they're only recommended at the bottom of stairs.
If you're going to put one at the top it needs to be screw fitted (with decent size screws and proper wall plugs) not a pressure fit one. Personally I'd also put it away from the top step if possible. Ours is actually a pet gate because it's higher than a standard baby gate so I felt it was safer. In our last house we didn't have any and just taught LO to go up and down the stairs safely. Our new (to us, very old) house has huge gaps between the bannisters so I was too worried about LO falling straight through, so we have gates top and bottom now
Depends I think. For me it was easier to put a stair gate at my daughter’s bedroom rather than the top of the stairs.