[Lal]

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.

Second question::

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.