回复：我能否自由进出加拿大

Yes, even your J1 visa is not multientry visa and has expired , you still can come back to US with that visa, valid I-797 and current DS-2019.

For entering Canada, the first time you only need landing paper, immigrant visa and your passport. But after that, landing paper is only good for travel with private vehicle,say, your own car. If you will take public vehicles like plane, bus, etc, you have to show your Maple Leaf Card at the port of entry.