About manually flying P-RNAV procedure - it is allowed only with GPS as a sensor. It is assumed that FTE (flight technical error) for manual flying is 0.8 nm, so with GPS precision you don't exceed 1 nm limit. Manual means no flight director, just HSI. With other types of sensors like DME/DME, VOR/DME or IRS, positioning error is more significant therefore at least flight director is required to significantly reduce FTE (source - Eurocontrol).
RF legs is a recommended option of P-RNAV specification (ref. JAA TGL 10) so at my opinion if on-board systems support RF legs, they may be flown as RNP-1 procedure.