If you take a continuous pay severance, unemployment would start when that ends, if you take a lump sum severance from your employer you can stare unemployment benefits a week after your job ends.
Yes, but tread carefully! If you file a claim for unemployment, you may forfeit your severance pay. Your former employer may attach any proviso he wants to the severance pay (per the IDES claims...
Yes, you can still collect unemployment benefits after receiving severance pay. These 2 benefits are separate and distinct from the other -- severance pay is the consideration given by the employer...
You cannot collect absolute retirement (meaning not returning to work) and unemployment benefits because the latter requires you to, among many things, continuously seek full time employment.