Can I set it to make sure garage is closed at a certain time WITHOUT IRIS's subscription from Lowe's?

Yes, the basic Iris service (free) will let you set schedules on the garage door opener.

With the free service from Genie, you would be able to tell from the app if the door is currently shut if not, then you use it to close the app. As the app is updated and users provide input, more robust features may be added in the future.