3, there are a few mechanisms you can use. (other than making one from scratch)
a, The mechanism off a stannah type stair lift.
b, The mechanism off a mobility scooter.
c, You can buy one that fits into a car that's swivels the car seat out, which would apply the same to a trike, But having never seen or used one of these? i do not know if it is manual or electric or how much space it needs?
d, Some Bus (drivers) seats also have a swivel mechanism, and you may be able to get these cheap from a specialist breakers, (also some industrial machinery must use them , such as forklifts etc. but you would have to do research on this.)