While most(?) beginners on mountain dulcimer start out with a noter, those interested in more than very simple use of the instrument invariably(?) graduate to "fingering" the frets.

As commonly held/used a noter can result in a lot of stress on the one finger that presses it down. If RA is a problem in the wrist, you might find it's also present - or "incipient" - elsewhere (i.e in the knuckles?), if you begin holding a noter in a fixed "grip" and constantly pushing down with it.

Some people find the actual pressures, forces, and positions less a problem with RA than activities that require some stress in a fixed position, and that's the common complaint with guitar where the wrist has to be positioned to wrap the hand around the neck. I'ts also fairly common with fiddle players, due to the fairly fixed "wrist arch" commonly used.

The "more motion" on something like a hammered dulcimer might actually be beneficial. Whole arm motions are required to reach the right strings, but the only "effort" comes from flicking the hammer against the strings, which is largely in the fingers. The wrists, where the pain complaint is indicated, can be fairly "fixed" or are somewhat free to flex more or less continuously without obvious effect on playing. (An experienced HD player may have other input on playing style, but it's academic if the instrument doesn't interest our victim present subject.)

Failing to consider another instrument due to unconfirmed fear of pain probably is not productive thinking. A fairly brief trial, perhaps on a borrowed instrument, will let you know whether the problem exists.

Failing to consider an instrument because "it's not something you'd want to play" is of course perfectly valid.

An autoharp might be another alternative, but would appear (to one who hasn't played one) to dictate something of a "curled wrist in mostly fixed position" somewhat akin to the guitar, although I'd think that probably the finger pressures may be somewhat less.

And of course there's always the harmonica if nothing else works - unless of course one is addicted to munching peanuts during sessions.

The lap dulcie looks like a perfectly appropriate choice in the present situation. Only personal experience will really tell if it works for the one who tries it.

John