Disengaged

1. Does "disengaged" mean "non-military persons like nurses"?

2. Does the green sentence imply "viewers who like such artworks probably will not love the works of artists whose paintings of the scenes of wars are sold for high prices and printed in military magazines"? I can not understand the meaning of "rivet-counters" in this context

Context:

The American John Singer Sargent’s Gassed of 1918–1919 – a massive canvas some two and a half by six metres and replete with bodies of the dead and wounded and of the living and disengaged– has a room to itself in London’s Imperial War Museum. Viewers who appreciate such pieces may not respond so favourably to the ‘rivet-counters,’ whose past and recent dramatic reconstructions of moments of ‘derring-do’ fetch high prices and grace countless military magazines and journals. This art, too, however, has a place in this book.