Does Costco do returns on seasonal potted plants?

Not sure, but their return policy is generous if the request is reasonable

I would definitely return a dead plant within a month, if purchased from Costco.

Costco has notoriously lenient return policy. No questions asked. I am sure you can.

Costco Return policy, no limit, except, electronics (90 days), cigarettes, alcohol.

I would say as long as it's not dead and within 30 days, most likely.