Upholstry is the work of providing furniture especially seats with padding, springs, webbing, fabric or other decorative things.
I think upholstry would be very hard work.
Yup, it actually is work. There are professionals. Theyre called upholterers and an apprentice upolsterer is sometimes referred to as an outsider or trimmer. its the real deal!
Yes, I have seen some really great work done by an upholster. 
The term upholder is sort of an old term for upholsterer but it sort of has a connotation of repairing furniture instead of decorating/creating new upholstere pieces from scratch.