It is very clear to me, that an external input triggers vipaka vinnana.
“Cakkhuñca paṭicca rūpe ca uppajjāti cakkhuviññāṇaṃ.”
Dukkhasamudayasutta (SN 35.106) says:
“…Cakkhuñca paṭicca rūpe ca uppajjati cakkhuviññāṇaṁ. Tiṇṇaṁ saṅgati phasso.
Phassapaccayā vedanā; …”
When vipaka vinnana is established “tiṇṇaṁ saṅgati phasso” happens. Only then “phassapaccayā vedanā” follows. How does “tiṇṇaṁ saṅgati phasso” happen? Samphassa is “contact with defiled gati”. Thus mind incorporates san/gati into the cittas with the help of manasikara/cetana cetasika. That requires suitable namarupa. I don’t understand why salayatana can be bypassed in this case. Is this not PS?
You said:
“A key point is that cuti-patisandhi takes place in a citta vithi that starts with an arammana coming to mind. Thus the starting point in PS is “vedana paccaya tanha.”
That seems not right. There is at least (sam)phassa-ja-vedana before vedana-ja-tanha.