On page 166:”Thus, according to Ledi Sayadaw, in the mind door
too there is a fourfold presentation of objects. The course ending with
registration can be called a very clear (ati-vibh³ta) presentation; the
course ending with javanas, a clear (vibh³ta) presentation; the course
ending with mind-door adverting, an obscure (avibh³ta) presentation;
and the course ending with mere vibration of the bhavanga, a very ob-
scure (ati-avibh³ta) presentation. The clarity of the presentation depends
on either the prominence of the object or the strength of consciousness”
So, according to that, there can be Citta Vitthi, where a dhamma comes in that look like:
B B B A V V V V V V V V V B B B B
Similar to page 161 table.
Now, a dhamma is incoming, but only investigating citta(the one that I proposed) is arising, then how can it be anything else?
In your post
You write:”The seventh akusala vipāka citta is called the investigating consciousness accompanied by equanimity (upekkha-sahagata santirana citta). This is the citta that is responsible for the birth in the apāyā (lowest four realms), i.e, it acts as the paṭisandhi citta for the birth in the apāyā.”
Yes, agreed, and in Bhikkhu Bodhis book, the investigation citta can arise as Patisandhi but also as “Bhavanga citta” (which is A and V in above diagram, not B because B is not a citta as you pointed out)