Commerce: Combining unwanted quantity discounts
Not really a good answer as such but at the moment (June 16) there is currently no facility to print the actual price a particular item will sell at after all discounts/sales are applied (unless you calculate it yourself).

This seems to me a bit of an oversight.

I have requested it as a feature, and Luke seemed amenable to the idea but I have no idea on the ETA for it.

To solve the cumulative discount issue - you can either express the discount as a cumulative thing as you mention, or you could write a simple plugin that listens to

https://craftcommerce.com/docs/events-reference#commerce_discounts.onbeforematchlineitem

...and only applies the desired discounts when the desired conditions are met