Final update: fully resolved. Turned out to be two separate issues layered on top of each other — the weak 5V supply (confirmed earlier), plus a ribbon cable causing intermittent SmartSerial comms errors (Timeout/Extra character/Serial Break, escalating to port shutdown after ~6 minutes of runtime). Replaced the ribbon cable with a direct DB25-to-DB25 connection between the 7I92 and 7I76 (board mounted directly on the 7I92's P2 socket) — 30+ minutes stable, zero faults. E-stop wired to TB6-1/input-00 and confirmed working through PathPilot's UI.
It's worth noting, the ribbon cable fault was probably the proximity of a switching power supply nearby with everything laying on a table.
Thanks again to all for the help throughout.
