We're going on Sunday first weekend. Should we be able to see every house with Express?
Yes, you should (I would say definitely, but you never know) be able to go through each house with Express. If you're new to the event, then I recommend trying to get there sooner than later. That way you have more time to figure out the flow of things. During the event you might want to spend some time in scare zones or you might just want to breeze through them on the way to the next house. If you want to watch one or both shows, then factor that in too. Same for if you want to go on some rides. If staying late (open to 1 am or 2 am depending on the day) is an option, then you have more room for everything. If you want to leave at an earlier hour, then you'd probably want to Stay and Scream or be there for gate opening.
We've been going since 2012 (always September to mid-October), and always had Express Passes. We almost always Stay and Scream, and we've never had a night where we couldn't go through each house if we wanted. We've been done by 10:00 and we've been done after midnight. Just depends on pace, crowds, and what we want to do. This includes 1-2 shows, 1-3 rides, a food break, and several drink breaks (and associated restroom breaks). Some nights we spend 30 minutes to an hour in Diagon Alley, since it's (usually) less crowded than during the day.
I don't want to make you feel like you have to get there and rush without enjoying everything, but I also don't want you to feel like you can do everything with no pace at all. If you want more advice, please ask and let us know what your priorities are, and we should be able to help you get the most out of your HHN w/ Express. Finally, don't fret, because you are going to have a great time!