[Tom Bunch]

I'm not sure I completely understand your question about headers, but here is a pile of words that may help clarify.

1) OmniPlan doesn't currently have support for 3 layers of date headers.
2) If you set your project for "Month" scale, you may optionally turn on a "summary scale" that will display the quarter containing 3 months. That sounds like what you're seeing.
3) the format of the summary header can be customized under OmniPlan -> Preferences, "Display Preferences" tab.

You didn't attach an actual image, but it sounds like you want day scale with a summary (given that you can't have two summaries).

So play with customizing headers for the "Day" scale. Maybe:
Summary Header Format: (Month) — W(Week of)
Date Header Format: (Day of Week) — (Day of Month)

As for horizontally scrolling, hidden scrollbars in recent years have definitely made this tougher to pick up on, but shift-scrollwheel should get you back and forth. Alternatively, you can hold down the space bar and pan-scroll by click and drag in any direction, even diagonally.

-Tom