Can I re-enter the USA from Canada or Mexico with just a green card?
Yes, all you need is a green card to cross into the USA. But you should absolutely carry a record of the name change with you while you have mismatched names and apply for a new Canadian passport immediately so that your documents have matching names. Theres not much that will raise suspicion of immigration fraud/fraudulent documents faster than documents that dont match up and not having evidence that supports the mismatch. The last thing you want to do is sit in a CBP office while they take their sweet ass time (hours) to ensure you are who you say you are and are legally permitted to reside in the USA.