Car won’t go past 20 mph, struggles to start?

I hired Your Mechanic On Call to fix an oil leak. Before the repair, my car ran fine except for the leak. The mechanic said I needed an oil pressure switch, alternator, and belt, but told me the alternator wasn’t urgent. I skipped it. He fixed the oil pressure switch and the serpentine belt.

After the service, my car won’t go past 20 mph, struggles to start, and all the dashboard lights are on( literally all of them, traction, brakes, engine etc...)Now he says it’s the alternator, but I’m not having battery problems, and three inspections, including AutoZone, say it’s fine.

Has anyone seen this before? What could be wrong?