I think the slide/fret question will have to be determined individually, depending on the condition of each person.

My friend Joe Adams played the guitar despite extreme rheumatoid arthritis crippling his hands as well as much of the rest of his body. He used an open tuning and a slide (I believe an ordinary Hawaiian guitar slide), and a capo of his own devising so that he could play in different keys without retuning. He used a Hawaiian guitar nut to raise the strings, and the capo had a piece under the strings as well as over them, to maintain their height above the fingerboard.

When Joe's condition got too bad to play the guitar, he played harmonica. A very inspirational guy -- reminiscent of Joan MacIsaac's song "When I Can't Play I Will Sing".