The brown edges almost certainly are from something in
the mount. The browning is probably from sulfiding of the
image around the mount. Lack of fixing or washing can cause
sulfiding but its usually visible everywhere although it may
not be even.
  I've observed slight yellowing of prints stored in
original paper boxes for 10 years or more. These boxes are
not intended to be archival. I don't know what could be in
foam core that could cause the problem but I am not an
expert about that. Its certainly not from dry mounting. The
mounting tissue is archival and seals the paper from the
rear (RC doesn't need it). 190F is low temperature tissue
suitable for B&W RC and color papers.
  It is just possible that there could have been some
penetration of solutions into the paper support at the
edges. The plastic coating does not extend over the edges
but this is not often a problem unless wet times are very
extended. Since the prints are dry mounted its hard to know
what to do about them. If you can remove them its possible
the stain could be removed by bleaching and redeveloping.
  For display prints on either RC or fiber its a good idea
either to treat the print with a stabilizer like Agfa Sistan
or tone it in a sulfiding developer. Sistan is not as good
protection as toning but does not affect the color or
density of the print. For slight toning with attendant
protection a Polysulfide toner like Kodak Brown Toner, Kodak
T-8 toner, or Agfa Viradon are suitable. These tone all
densities evenly so will proved significant protection with
even slight toning. Gold toner is also a good protectant but
is too expensive for routine use.
 When a print is on display it is subjected to all sorts of
polutants in the air which can attack the image, even if the
mounting materials are safe. Modern RC paper is probably as
stable as fiber base but life in the big city is not healthy
for either so the protective solutions become necessities.