How do we know a single differential/linear trail dominates the others?
The short answer is that we don't. There are several block ciphers (most famously PRESENT) where multiple linear trails combine to form a stronger distinguisher. The collection of trails is called a linear hull. The concept has been extended to differential-linear cryptanalysis, but I don't have any reference for a &quot;difference hull&quot;. These multiple trail effects seem to be more pronounced in lightweight designs.
The analytic approach seems to be to assume that hulls don't contribute significantly unless they can be demonstrated to do so. If the term &quot;resistant&quot; is used rather than &quot;immune&quot;, then this assumption does not feel too unreasonable.