Issue with Power BI Capacity Unit (CU) Overuse and Dataset Optimization

Lots of things to unpack here

 is there a way to understand how CUs are utilized during and after a dataset refresh? 

CUs are incurred by a combination of duration and computational complexity. This includes both Power Query transforms and calculated columns and tables.  You can have a long running refresh that doesn't cost much, and a short refresh with ginormous computational cost.

how CUs are distributed or calculated between background and interactive processes

The main difference is the smoothing period - 24 hrs for background, and (most of the time) 5 minutes for interactive.

moving refreshes to off-peak hours

That has very limited benefits, as the smoothing period is the same 24 hrs no matter when your run the refresh. It is also not meaningful in an enterprise environment operating 24x7.

To emphasize again:  Reducing the schedule is important, but not the most important.  The most important is to reduce both the runtime and the complexity of each refresh.  Your primary KPI should be the cumulative CU consumption per developer per day.  Optimization of interactive queries would be a secondary KPI.