Dominant in legalize has a slightly different meaning. Especially when often akward or even weird jurisprudence comes into play. You could for example wager that if Amazon's position in market A is strong enough to dictate a dominant position in market B; this would make them a "dominant actor" in market A per definition and intent of the legislators. As such, any court could reasonably conclude that Amazon needs to be sodomized.

Less than half, and if you look at "retail" it's a single digit.

Not usually enough to get hit on monopoly measures!

It's fairly easy to breach ( international ) competition law and suffer the consequences without even coming remotely close to monopoly talk.

Sure, but you can do that without being dominant at all! When it specifically comes to the issue of dominance, I'm not sure if Amazon is really there.

Dominant or not dominant is not the main issue; whether or not Amazon is abusing it's market power is. For a fact we know that force is exerted by Amazon from a different market resulting in a strategical advantage for Amazon and a heap of deadweight loss; something that ought to be illegal in all shapes and forms when created intentionally & mostly is. ( https://en.wikipedia.org/wiki/Deadweight_loss )

IIRC 40% is enough in most markets to have “monopoly” charges stick. I know, pretty low bar..