For page formats that don't include the Back/Next arrows, you can make your own buttons that you can apply to your Parent page that have Go To Next Page and Go To Previous Page actions applied for the navigation.
I noticed in your link that your content has Desktop Scaling/Best Fit applied. The Continuous Scroll (Horizontal) page format can left align content even with scaling applied.
Otherwise, to left align the content you would need to remove Desktop Scaling and apply custom CSS in the Resources section of the Export HTML5 with in5 dialog.
Another option would be to export with the page format set to Liquid Layout--even if you don't have Liquid Layout rules applied. The Liquid Layout page format can include Back/Next arrows but won't scale--even if Desktop Scaling is applied.
You could also try setting Desktop Scaling to Fill Width so that it stretches to both the left and right margins.