Good question in #1.
There, chaṭṭhāyatana refers to just the mind (sixth ayatana).
As explained in that post, this is about how a vipaka vinnana arises.
– The arammana is grasped by the mind.
– As you can see, when that vipaka vinnana comes in, the PS process starts at “saṅkhāra paccayā viññāṇaṃ” and NOT “avijjā paccayā saṅkhāra”
If one’s mind decides that the arammana is “enticing” or “distasteful” (at the vottapana stage; see #15) then it will start doing sankhara possibly with other ayatana as well. That process will start as “avijjā paccayā saṅkhāra” and will get to salayatana (instead of just chaṭṭhāyatana).
– I just revised #18 of that post to emphasize this point.
2) In the case of a uppatti PS cycle, is it possible for a single cycle to span more than 2 jati (births) or even more than 2 bhavas?
No. Each uppatti PS cycle leads to ONE bhava.
– Of course, it may just provide more strength to an existing bhava that matches the sankhara that one is doing in that process.