Do part-time employees usually get offered severance packages or is this (kinda ghoulish) business as usual?

by design part-time employment usually does not confer the benefits of full employment, no. that's part of why so many unions seek to either upgrade part-time workers to full time workers or limit the number of part-time/subcontracted workers that a company is employing--otherwise you basically have an underclass of people who can get totally fucked at any time.