First of all, all will never attain Moksha simultaneously, as that is against the will of the Supreme Who created this world. As Sri Krishna puts it in the Gita:
7.3 Among thousands of men a rare one endeavours for perfection. Even of the perfected ones who are diligent, one perchance knows Me in truth.
Moksha is for the one who knows Him in truth.That is always very rare.If all are able to climb Mount Everest, the glory of climbing will die!
Sri Sri Devi-Mahatmya also says almost the same thing:
tathApi mamatA-avarte moha-garte nipatitAh/mahAmAyA-prabhvena samsara-sthitikArinA../jnAninAm api chetAmsi devi bhagavati hi sA/ valad Akrishya mohAya mahAmAyA prayachchati/ tayA visrijyate visvam jagad etat charAcharam/ saishA prasannA varadA nrinAm bhavati muktaye// (chapter 1, mantras 53-57
All the beings are still made to fall in the holes of delusion and whirlpools of 'mine-ness' by the world-preserving effect ot Mahamaya. She forcibly fills even the minds of the wises by attractions and delusions. She alone can grant liberation only when She is pleased and is in the boon-giving mood.
Vishnu and Vishnu-Maya are just two sides of the same coin as per our scriptures, as Shakti and ShaktimAn are one and the same.
All will therefore never be allowed to get Moksha simultaneously, as it will be against the will of the world-preserving power of the Almighty.
Now about the Pitri-rina.No, He who surrenders Himself to God completely pleases Him by devotion. Sri Krishna Himself says in the Gita that He is always pleased with His devotees.
The mantra says
priyatAm pundarikAkshah sarva-jajnesvaro Harih/ tasmin tushte jagat tushtam prinite prinitam jagat//
May the lotus-eyed Lord, who is goal of all the worships be pleased.If He is satisfied, the entire world is satisfied. If He is pleased, ALL are pleased.
As ALL are pleased, the real devotee is no more bound by any obligations to any party including the manes. So he is automatically freed from the pitri-rina and does not have to procreate any longer.
But as mentioned, that always happens for a chosen few. All will never get Moksha simultaneously, even at the time of pralaya.