Kings have abdicated for love. Whole wars started over infidelity. Presidents and ministers disgraced over affairs.

I can't think of a region or era that doesn't have examples.

A trade route isn't just a road or a trail. People don't typically start at one end and travel all the way to the other.

It's a network. So on major routes there would be resources and services that define it as said route.

Also the goods almost always changed ownership many times as they were traded along the route.

>did they do so willingly, rather than raise a relatively unproductive laborer?

It's that one.

They are all steppe tribes focused on pastoralism (keeping herds) and fighting with (mostly) bows from horseback.

Generally it's dictated by whoever was there when a colonizer showed up.