“Is the body or the gandhabba which is constantly changing or is it both?”
– It is both. Of course, moment-to-moment changes are not discernible.
“And is the PS process happening continuously as we are changing continuously 24/7 or does it cease to happen sometimes (especially when we are in a deep sleep)?”
– All moment-to-moment changes are not necessarily due to Paticca Samuppada (PS).
– Such changes can happen in real-time due to PS processes (Idapaccaya PS), but such changes ALWAYS happen because both our physical bodies and gandhabba are saṅkhata. Everything in this world is a saṅkhata that is constantly undergoing changes and destroyed at some time.
Only akusala-mula PS processes have “avijja paccaya sankhara”.
– If you start doing vipassana that is not an akusala.
– You have skipped the step “sankhara paccaya vinnana” in your analysis.
Try to understand how akusala kamma are initiated by the mind via sankhara;
“Saṅkhāra – What It Really Means”
Also understand the difference between Kusala-mula and Akusala-mula PS processes in the PS section.