2021 Chevy Malibu; Automatic; 40K miles; OBD-II unknown

Vehicle stopped operating on road, power cut-off.  Vehicle was towed to nearest chevy dealer and diagnostic revealed oil sludge in engine. Dealer dissasembled to reveal valves*. As far as I know that was the extent of their work. Vehicle was towed home. A DIYer cleaned the valves, changed the oil, changed the oil filter, changed spark plugs, cleaned the oil pump, cleaned fuel lines, replaced fuel filter, replaced vaccuum pump because it had a broken internal piece. Assembled car back together, now car cranks but no start. Checked all fuses, breakers, switches, sensors, battery, wiring, etc.. Unsure if codes are present.  Owner n dealer are at an impasse. Figure warranty no longer applies??? Any possible solutions? Good service manuals?

*All accompanying parts for access also removed.

Vehicle used primarily for Uber, etc...