The EVIC msg you have post update has to be associated with the supplemental cabin heaters - there's three of em. The rough running/chugging is, anecdotally, normal for these engines during the first couple or three minutes of operation when the OAT gets close to freezing, especially if the truck has a single alternator. Consider that the grid heater is drawing upwards of 200A when it cycles on. Then there's those supplemental cabin heaters trying to work too. And, if it's cold enough, the fuel filter heaters and DEF line heater will be asking for juice. And all that doesn't include the cabin demands for power (HVAC blower, heated seats/steering wheel, radio etc). Watch the volt meter after a cold start. The voltage will swing back and forth from the low 11s to mid 13s as the grid heater cycles. The rough running/chugging is simply the engine's attempt to maintain idle speed while the batteries are being pounded by the massive elect load.
Auto activation of high idle is dependant on OAT and coolant temp. It the engine is started at OATs of about 20F or below and without the benefit of a block heater, the idle will increase to 1000 RPM shortly after startup. If the block heater has been plugged in for a few hrs, the engine will initially idle at about 650 RPM and increase to high idle after about two minutes. With an OAT of about 20F or higher and with no block heater assist, the engine will initially idle at 650 for the first two minutes. Below 0F, and regardless of coolant temp, the engine will always idle at 650 for the first two min of operation.