The RNP requirement that you refer to is an Oceanic requirement that you will likely never exceed unless you fly an ACJ. It use to apply more often when the A320 family was not equipped with GPS Primary.
In short, RNP-10 it limits the Navigation without ground based NAV's to 6.2 hrs from ground alignment and to 5.7 hrs from inflight update with a ground based NAV. This limits go to RNP-4 with GPS Primary.
With the range of the A320 family, except the ACJ, it is very hard to exceed this limitations. We took them into account during North Atlantic ferrys, but never came close.
As far as the paragraph on the mode of entry in the MCDU PROG page, it is a technicality; it mentions the Radial Equivalent as per certain regulations, but the common way of entering it would be 10NM or 4 NM.
The "normal" radials we refer to a VOR do not apply here, it is just RNP.
Hope it helps!
G