hi Jai_star, first of all what you did on cutting the marshy roots away was right, but there is a few things ....
1) did u mount it immediately after trimming the roots?
2) IMHO, the hemp string and the sphag moss seems too "thick" then the roots might not breath well
i attached several pics to help out but this is what i usually do...............
my usual practice after trimming marshy roots away is to then treat with fungicide, then leave it aside in a well lighted aerated place but away from direct sun and heat for 3 days or so to let the "cure" the roots.
then i will mount it and line it sparingly with sphag moss allowing space for the roots to breath, occasionally i will add a chunk of sphag moss at the top of the mounted piece to provide additional water but not to drown the roots as they do not have direct contact with the roots as in pic 1 (up most portion of the pic)
hope these opinions answer some of your queries