I have always been under the impression the f. microcarpa is the latin name for f. retusa.
I have never seen a ginseng ficus specimen, but that doesn't mean they don't exist. One of the reasons that I avoid the ginseng ficus is because of the reverse taper, I think that its their natural growth pattern or something. Depending on how big the tree is, you could try cutting across the widest part of the bulge and treating the tree like a cutting then. If it works, you will eliminate the reverse taper and develop a spectacular nebari in time.
here's a link that discusses this issue:
http://www.bonsaisite.com/fatfig.html
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