Yes, ppbus MUST be stable and present. I would make sure you soldered the nands properly/completely. If DFU completed successfully, the nands should be ok. So, I would also suspect and inspect the SSD controller ICs. Remove the bridge resistor for PPBUS and SSD power to see if ppbus stabilizes. If so, then you have an issue with the SSD power ICs, but again, that would typically prevent DFU restore from completing.

You "MIGHT" have a damaged T2; confirm all SLP/AWAKE voltages are stable and not pulsing after 20 seconds.

Yes correct, it must be programmed. It's still part of the "old" SSD at this point.

You need to focus on DFU mode right now; if you can't enter DFU, we need to figure out why.

What are your T2 voltages? SLPS2R & AWAKE....

Received this unit for data-recovery only. Unit has no power and charger cycles the instant it boosts to 20v on all ports. Unit will show life when REVIVING, but fails DFU eventually (expected DFU mode, i.e. a cycle/restart of SOC). Board and unit are super clean, I suspect the unit may have simply lost power at a critical point of update. Customer doesn't care about repair, only wants data. But I strongly feel a restore is what the unit needs. Again, during revive, video will show on LCD, but there is never any trackpad click (I'm unsure if there is normally anyway at this specific point, reviving...

A1990 and A1707 batteries not cross-compatible. Just mentioning that because you mentioned them both as if they might be.

Hey this is way way late, but I randomly came across this thread: These boards used a ton of different T2 chip models, and they varied by RAM cfg. The 16GB boards use a different T2 model. If you upgrade the RAM, you have to upgrade the T2, which is of course not worth it in the vast majority of cases.