On Thu, 21 Jun 2018 13:19:02 wrote:

Over the last 50 years it has developed a bend in the plane of the
diagonal between the corners. ie this meant that one corner was
about 3" from the wall when the other three corners were touching the
wall

you did not say what the problem was so far

you did mention the symptoms

has the wood warped or has the canvas shrunk

maybe humidifying the canvas would allow it to relax and then some
stiffeners could be attached


there are some good sites about art restoration

iirc some museums have good sites for this


or you could wait some and then you may not care anymore

or you could declare it to be unsalvageable as a way to have new
art in that place