Dunking Hands

The Toras Kohanim derives that a mikvah causes taharah only if it has 40 seah but a natural spring, a מעין  doesn’t require 40 seah and it causes taharah even with a כל שהוא.  However that halacha of a מעין  isn’t applicable for a human being because the whole body has to be covered.  the Gemorah (Chagigah 11a) says that 40 seah is required for a mikvah because that is the amount of water necessary to completely cover a person’s body.  That is measuring with the average person and therefore by a mayen we may measure with the specific person dunking.  However, the Rosh and R”i hold even in regard to a mayen 40 seah are required (see Tur Yoreh Deah beginning of siman 201 cites all the opinions.) 

Rav Shimon Shkop (Sharei Yosher 3:19) says that the 40 seah requirement for a mikvah isn’t an amount of water necessary o be contained in the mikveh to cause taharah, it’s necessary for the shem mikvah as opposed to the 40 seah requirement for a mayen is just an amount of water needed to cover the person dunking. 

The Shulchan Aruch Orach Chaim 159:14 brings two opinions if for netilas yadayim one needs to dunk his hands in a mikvah of 40 seah or its enough if it has enough water to cover his hands.  However, by a mayan everyone agrees 40 seah isn’t required.  If 40 sea is only necessary because that’s the amount of water needed to cover a person’s body, when it comes to washing only one’s hands, 40 seah shouldn’t be required.  So why is there an opinion that 40 seah is required?  Furthermore, what’s the difference between the mayan and mikvah? 

It would seem they hold that 40 sea is required for the shem mikvah and therefore require 40 seah even for cleaning just one’s hands.  However, in a mayan there is a chalos of causing taharah even with less than 40 seah.  The 40 seah requirement is just to cover the person's body but for hands 40 seah wouldn't be required as Rav Shimon explained (Yalkut Yosef.)  However, this explanation is difficult for it assumes that there is an opinion that there can’t be a mikvah with less than 40 seah but we see that to dunk utensils that are small it is enough with a רביעית (see Pesachim 17b and Rashi there;) seemingly one can have a mikvah with less than 40 seah?  However, the Gemorah comes out (Nazir 38b) that the Rabbis always required 40 seah for a mikvah so it can be that same enactment applies to washing hands.  If so, why is there an opinion that 40 seah isn’t requires for washing hands, what happened to the Rabbinic decree?