There isn't anything complex about "understand[ing] the risks of mutual fingering". There is no STD risk to speak of. You are correct that theoretically such transmission could occur, for the reasons you state. But it is exceedingly rare if it occurs at all. In 30+ years in a busy STD clinic, I don't think I have ever seen a case of any STD that did not involve insertive vaginal, anal, or oral sex -- and certainly never say any infection in someone whose only apparent risk had been fingering or mutual masturbation.
As for your request to "describe in detail", little more is known than I just said. Because such transmission appears to be rare, there has been no research on it -- and I doubt there ever will be. Here is a thread that describes the biology of STD/HIV transmission, which may help explain things for you. Read the entire thread; the most important information comes in follow-up comments:
http://www.medhelp.org/posts/show/1119533
Regards-- HHH, MD