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Would it be feasible to power off the ARM core on the modem and do everything from the main PinePhone ARM SoC?

I think the ARM core on that is pretty weak even compared to the A64 (and certainly compared to the RK3399 of the PPP!). So while possible, I think it is pretty infeasible. That and I think everything is wired to the main CPU anyways.

How feature complete is it compared to the proprietary firmware?

Just as feature complete.

Has anyone looked at the ADSP firmware part of the modem?

Aside from clean room reverse engineering, I do not think there is anyway to actually replace that. One can wipe and reformat the ADSP, but to my knowledge, if the source code is out there, it is certainly leaed proprietary source code.