Yes, MP is mandatory. Yes, a seller can refuse to sign up for managed payment, if the seller doesn't want to continue selling on eBay anymore. Sellers will no longer be able to sell on eBay without MP. eBay has been promoting the advantages for the past 18 months or so ... there will be no "MP versus staying with PayPal ". Staying with PayPal is not an option.
https://www.ebay.ca/sh/fin/dlp