Can I put new leather on them?

Replacing the covering is standard practice when the original fails. An upholsterer will save the underlying horsehair. Request traditional brass nailing rather than modern staples to secure the new skin.

My husband wants to use one at his desk every day for working from home, but are they actually comfortable for computer work?

Most were built for reading rather than typing. The seat height often sits lower than a modern office chair. The back usually angles away from the desk. They work well for reading documents. If you need to type for eight hours, the lack of lower back support might cause fatigue.

What is a metamorphic chair?

It is seating that hinges open to become a short set of steps. The transformation usually involves releasing a small brass catch.

We have quite a small study with narrow doorways in our cottage, do the legs on these old chairs ever unscrew for moving them?

The legs on Georgian and Victorian pieces are nearly always fixed rigidly into the frame. They rely on tight glued joints to stay stable. You will need to manoeuvre the whole frame through your doorway. Tilting it sideways and hooking the back through first usually works for tight spaces.