Waheguru Ji Ka Khalsa...Waheguru Ji Ki Fateh..........
As far I know and IMHO...Mool Mantar is Up to "Gurprasad".. because after that JAP starts...it doesn't mean to recite...but It means JAP baani Starts...Like Rehraas, Jaap, Sukmani, Anand are baani's... We added Ji and Sahib to JAP baani... as a respect...and we added Sahib as respect to all baani's...
now ..let me say something to support it...(these are given as support to me)....
Many times in Sri Guru Granth Sahib... Whole Mool Mantar is written ..and that is up to "Gurparsad" (sorry I don't remember the count)...
and many times Guru Ji had cut short it to "EK OngKaar Gurparsad"...or "Ek Ongkaar Satnam Gurparsad".... like we recite first 5 Pauri's and 40th Pauri of Anand baani (Anand Sahib- with respect)
You will find "AAD SACH...JUGAD SACH...HAI BHI SACH..NANAK HOSI BHI SACH" in Sukhmani baani (Sukhmani Sahib) ...its same like "AAD GUREY NAMAH...JUGAD GUREY NAMAH..SATGUREY NAMAH..SRI GURDEVEY NAMAH" in Sukhmani Sahib..while starting Sukhmani Baani...
This is compliation of what I heard in Gyani Amreek Singh Ji's katha broadcasted from Gurdwara Dukh Niwaran Sahib, Ludhiana, Khalistan....last month... and a talk with Raagi of Gurdwara from Bradrock road, farifax, Virginia.
Now what I practice....., I believe in reciting more baani...so I recite upto "Nanak Hosi Bhi Sach"...like I recite Long Rehraas ...Long Chaupi Sahib..sometimes full Anand Sahib while doing Rehraas (Sahib)... its just what I think to made a habit of reciting more baani...but if I will be doing in Gurdwara.. I will do as per SGPC.short Rehraas (Sahib)..short Chaupai (Sahib)...6 Pauriyaan of Anand (Sahib) baani...and no Pauri.at last....because I don't want to confuse others with my practice...
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-sikhi-sikhism/25273-mool-mantar.html
I hope this will help to clear you doubt...Actually Mool Mantar is up to Gurparsad...and we have practice that if we are doing it in Sangat...because that's what is true.and we are not supposed to divert the Sangat..but individually you are free to recite baani to any extent you want..may be up to full 1st Pauri of Jap (Jap Ji Sahib)...
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=25273
This is what I learned and what I feel...
Chardi Kala.....